[Hampshire] LAMP performance

Just been asked by a client to measure his customers system peformance limits
by "load testing" his server. They customer wants "real world" measurements and
the budget is nigh on zero. So I suggested using the log files to build a
site profile and then use some tool to hit the server with requests of a similar
composition to the profile gleaned from the logs.

Not perfect by any means but I could not find anything that would mimic real
user behaviour, provide reporting and would scale to say 600 concurrent "users"
without hammering the box used to do the testing.

The only tool I am familiar with is apache 'ab' or a bunch of perl based systems
that are too resource intensive for this job.
The problem with ab is that it only takes one URL at a time and from the source code
it is not possible to modify it to handle more than one URL at once.

So any suggestions for a tool that could be given data about a set of URL (requests)
and hit counts and using a set cuncurreny limit process the URLS and provide raw
results I could examine?

My current plan of setting off a number of concurrent 'ab's (one per URL) is not ideal.

Jacqui