Dear all
Thanks to everyone who replied on this one.   I've had several independent 
problems over the last few days including a lighting problem and a complete 
XP crash (blue screen) so I've lost the original messages - hence I can't 
just reply.   Back to the plot - we were at the stage of me posting my 
xorg.conf and xorg.0.log.   Both of these are given below.
I'm totally out of my depth here I'm afraid.   According to xorg.0.log, 2 
cards seem to have been detected - one which is nvidia 6100 (which is not 
the card I have inside so I assume that's the onboard).   The other - which 
should be an 8400 seems to have been misdetected "chipset unknown".   I've 
had a fiddle with org.conf using vi but I'm still getting errors.
To be honest, I don't particularly mind which card it uses - though I had 
hoped to run KDE or maybe compiz-fusion so it would be good if the better 
card is the one detected.   The irony of this is that I played with FC8 a 
few weeks before Christmas and it couldn't detect the onboard graphics of 
that machine (different from this) so I deliberately bought the 8400 
thinking it was more likely to be detected by the installer.   Now it seems 
my best shot may be to use the onboard!!!
Lastly - Sean asked which mobo I have and it's WinFast mcp61pm2ma
xorg.0.log and xorg.conf files follow
Cheers
Rob
--------------------------------------------------------------------------------------------------------------------------------
X Window System Version 1.3.0
Release Date: 19 April 2007
X Protocol Version 11, Revision 0, Release 1.3
Build Operating System: Fedora Core 7 Red Hat, Inc.
Current Operating System: Linux Warne 2.6.23.1-42.fc8 #1 SMP Tue Oct 30 
13:55:12 EDT 2007 i686
Build Date: 17 October 2007
Build ID: xorg-x11-server 1.3.0.0-33.fc8
 Before reporting problems, check 
http://wiki.x.org
 to make sure that you have the latest version.
Module Loader present
Markers: (--) probed, (**) from config file, (==) default setting,
 (++) from command line, (!!) notice, (II) informational,
 (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Wed Jan  2 13:14:20 2008
(II) Open ACPI successful (/var/run/acpid.socket)
(II) Loader magic: 0x820b5c0
(II) Module ABI versions:
 X.Org ANSI C Emulation: 0.3
 X.Org Video Driver: 1.2
 X.Org XInput driver : 0.7
 X.Org Server Extension : 0.3
 X.Org Font Renderer : 0.5
(II) Loader running on linux
(II) LoadModule: "pcidata"
(II) Loading /usr/lib/xorg/modules//libpcidata.so
(II) Module pcidata: vendor="X.Org Foundation"
 compiled for 1.3.0, module version = 1.0.0
 ABI class: X.Org Video Driver, version 1.2
(--) using VT number 7
(II) PCI: PCI scan (all values are in hex)
(II) PCI: 00:00:0: chip 10de,03ea card 10de,cb84 rev a1 class 05,00,00 hdr 
00
(II) PCI: 00:01:0: chip 10de,03e0 card 105b,0d15 rev a2 class 06,01,00 hdr 
80
(II) PCI: 00:01:1: chip 10de,03eb card 105b,0d15 rev a2 class 0c,05,00 hdr 
80
(II) PCI: 00:01:2: chip 10de,03f5 card 105b,0d15 rev a2 class 05,00,00 hdr 
80
(II) PCI: 00:02:0: chip 10de,03f1 card 105b,0d15 rev a2 class 0c,03,10 hdr 
80
(II) PCI: 00:02:1: chip 10de,03f2 card 105b,0d15 rev a2 class 0c,03,20 hdr 
80
(II) PCI: 00:04:0: chip 10de,03f3 card 0000,0000 rev a1 class 06,04,01 hdr 
01
(II) PCI: 00:05:0: chip 10de,03f0 card 105b,0d15 rev a2 class 04,03,00 hdr 
80
(II) PCI: 00:06:0: chip 10de,03ec card f05b,0d15 rev a2 class 01,01,8a hdr 
00
(II) PCI: 00:07:0: chip 10de,03ef card 105b,0d15 rev a2 class 06,80,00 hdr 
00
(II) PCI: 00:08:0: chip 10de,03f6 card 105b,0d15 rev a2 class 01,01,85 hdr 
80
(II) PCI: 00:09:0: chip 10de,03e8 card 0000,0000 rev a2 class 06,04,00 hdr 
01
(II) PCI: 00:0b:0: chip 10de,03e9 card 0000,0000 rev a2 class 06,04,00 hdr 
01
(II) PCI: 00:0c:0: chip 10de,03e9 card 0000,0000 rev a2 class 06,04,00 hdr 
01
(II) PCI: 00:0d:0: chip 10de,03d1 card 105b,0d15 rev a2 class 03,00,00 hdr 
00
(II) PCI: 00:18:0: chip 1022,1100 card 0000,0000 rev 00 class 06,00,00 hdr 
80
(II) PCI: 00:18:1: chip 1022,1101 card 0000,0000 rev 00 class 06,00,00 hdr 
80
(II) PCI: 00:18:2: chip 1022,1102 card 0000,0000 rev 00 class 06,00,00 hdr 
80
(II) PCI: 00:18:3: chip 1022,1103 card 0000,0000 rev 00 class 06,00,00 hdr 
80
(II) PCI: 01:06:0: chip 1106,3044 card 105b,0d15 rev c0 class 0c,00,10 hdr 
00
(II) PCI: 02:00:0: chip 10de,0422 card 174b,4001 rev a1 class 03,00,00 hdr 
00
(II) PCI: End of PCI scan
(II) PCI-to-ISA bridge:
(II) Bus -1: bridge is at (0:1:0), (0,-1,-1), BCTRL: 0x0008 (VGA_EN is set)
(II) Subtractive PCI-to-PCI bridge:
(II) Bus 1: bridge is at (0:4:0), (0,1,1), BCTRL: 0x0204 (VGA_EN is cleared)
(II) Bus 1 I/O range:
 [0] -1 0 0x0000c000 - 0x0000c0ff (0x100) IX[B]
 [1] -1 0 0x0000c400 - 0x0000c4ff (0x100) IX[B]
 [2] -1 0 0x0000c800 - 0x0000c8ff (0x100) IX[B]
 [3] -1 0 0x0000cc00 - 0x0000ccff (0x100) IX[B]
(II) Bus 1 non-prefetchable memory range:
 [0] -1 0 0xfdf00000 - 0xfdffffff (0x100000) MX[B]
(II) Bus 1 prefetchable memory range:
 [0] -1 0 0xfda00000 - 0xfdafffff (0x100000) MX[B]
(II) PCI-to-PCI bridge:
(II) Bus 2: bridge is at (0:9:0), (0,2,2), BCTRL: 0x001c (VGA_EN is set)
(II) Bus 2 I/O range:
 [0] -1 0 0x0000b000 - 0x0000b0ff (0x100) IX[B]
 [1] -1 0 0x0000b400 - 0x0000b4ff (0x100) IX[B]
 [2] -1 0 0x0000b800 - 0x0000b8ff (0x100) IX[B]
 [3] -1 0 0x0000bc00 - 0x0000bcff (0x100) IX[B]
(II) Bus 2 non-prefetchable memory range:
 [0] -1 0 0xf6000000 - 0xf9ffffff (0x4000000) MX[B]
(II) Bus 2 prefetchable memory range:
 [0] -1 0 0xe0000000 - 0xefffffff (0x10000000) MX[B]
(II) PCI-to-PCI bridge:
(II) Bus 3: bridge is at (0:11:0), (0,3,3), BCTRL: 0x0004 (VGA_EN is 
cleared)
(II) Bus 3 I/O range:
 [0] -1 0 0x0000a000 - 0x0000a0ff (0x100) IX[B]
 [1] -1 0 0x0000a400 - 0x0000a4ff (0x100) IX[B]
 [2] -1 0 0x0000a800 - 0x0000a8ff (0x100) IX[B]
 [3] -1 0 0x0000ac00 - 0x0000acff (0x100) IX[B]
(II) Bus 3 non-prefetchable memory range:
 [0] -1 0 0xfde00000 - 0xfdefffff (0x100000) MX[B]
(II) Bus 3 prefetchable memory range:
 [0] -1 0 0xfdd00000 - 0xfddfffff (0x100000) MX[B]
(II) PCI-to-PCI bridge:
(II) Bus 4: bridge is at (0:12:0), (0,4,4), BCTRL: 0x0004 (VGA_EN is 
cleared)
(II) Bus 4 I/O range:
 [0] -1 0 0x00009000 - 0x000090ff (0x100) IX[B]
 [1] -1 0 0x00009400 - 0x000094ff (0x100) IX[B]
 [2] -1 0 0x00009800 - 0x000098ff (0x100) IX[B]
 [3] -1 0 0x00009c00 - 0x00009cff (0x100) IX[B]
(II) Bus 4 non-prefetchable memory range:
 [0] -1 0 0xfdc00000 - 0xfdcfffff (0x100000) MX[B]
(II) Bus 4 prefetchable memory range:
 [0] -1 0 0xfdb00000 - 0xfdbfffff (0x100000) MX[B]
(II) Host-to-PCI bridge:
(II) Bus 0: bridge is at (0:24:0), (0,0,4), BCTRL: 0x0008 (VGA_EN is set)
(II) Bus 0 I/O range:
 [0] -1 0 0x00000000 - 0x0000ffff (0x10000) IX[B]
(II) Bus 0 non-prefetchable memory range:
 [0] -1 0 0x00000000 - 0xffffffff (0x0) MX[B]
(II) Bus 0 prefetchable memory range:
 [0] -1 0 0x00000000 - 0xffffffff (0x0) MX[B]
(--) PCI: (0:13:0) nVidia Corporation GeForce 6100 nForce 405 rev 162, Mem @ 
0xfb000000/24, 0xd0000000/28, 0xfc000000/24
(--) PCI:*(2:0:0) nVidia Corporation unknown chipset (0x0422) rev 161, Mem @ 
0xf8000000/24, 0xe0000000/28, 0xf6000000/25, I/O @ 0xbc00/7
Missing output drivers.  Configuration failed.
==============================End of 
xorg.log===================================
Here comes xorg.conf
# Xorg configuration created by pyxf86config
Section "ServerLayout"
 Identifier     "Default Layout"
 Screen      0  "Screen0" 0 0
 InputDevice    "Keyboard0" "CoreKeyboard"
EndSection
Section "InputDevice"
 Identifier  "Keyboard0"
 Driver      "kbd"
 Option     "XkbModel" "pc105"
 Option     "XkbLayout" "gb"
EndSection
Section "Device"
 Identifier  "nvidia GeForce 6100"
 Boardname   "nv"
 Busid       "PCI:0:13:0"
 Driver      "nvidia"
 Screen     0
EndSection
Section "Screen"
 Identifier "nvidia GeForce 6100"
 Device     "Videocard0"
 DefaultDepth     24
 SubSection "Display"
  Viewport   0 0
  Depth     24
 EndSubSection
EndSection