I use VM's all the time including software development purposes.
I have TWO servers which run VM's. These are:-
1) My ALWAYS ON Server. This is an Intel CPU Mac Mini, 2Gb Ram 80Gb HDD 
+ 250GB via firewire.
    This runs  VM #1  Webserver, Wiki, SVN & Bugzilla
                    VM #2  Email Server (Axigen)
  The O/S for all three is CentOS 5.1. I use VMWare Server so that I can 
move the VM's as required to Linux, Windows & OS/X.
2) My Dev Server. This is an Intel Q6600 Quad Core, 8GB Ram, 2TB disk.
 O/S is CentOS 5.2.
 It is currently running 4 VM's.
  a) Windows Server 2003 2Gb Ram 80Gb disk
  b) RHEL 5.2 2Gb Ram, 64Gb Disk
  c) Windows XP 512Mb Ram, 16Gb disk
  d) Fedora 9 1Gb Ram, 12Gb Disk
   This is shutdown a the end of the day. I snapshot the VM's as requred 
and back them up to my QNAP NAS.
This may be of use to you in deciding how you configure your 
environment. The Mac Mini  is quiet, draws not a huge amount of power  
so it an ideal candidate for 'an always on' server. The other beast 
while reasonably quiet, draws a lot more power than the former.
Stephen D