There are two 'gotchas' to look for: disk-controller and processor.
Whitebox HCL:
http://www.vm-help.com/index.html
If you have a 64bit processor with VT, you can run VMware 4x (or 5).
A 32 bit processor (OR a 64 bit without VT) can run VMware 3.x.
VT = Virtualization Technology
I was disappointed as I have a Dell PC that has an Intel DuoCore 64bit processor that DOES NOT have VT, therefore it can run VMware 3.5, not version 4 (or 5).
Note too that some SATA-RAID controllers do not work with VMware at all.
If you want to do raid, it won't work with software-only raid (like many Dell PCs)
http://www.vm-help.com//esx40i/esx40_whitebox_HCL.php
At home I run ESX3i 3.5 (32 bit) on an ancient Dell Server with 3GHZ CoreDuo processor.
I've got five instances of Windows XP, One Ubuntu 10, one MineOS (Crux UNIX) all run like a champ on 4Gigs of ram (seriously). (though the MineOS can peg the processor at times).
An optimized XP instance will consume maybe 5% of the total processor and maybe 300KB of RAM. Win2008 is about the same, processor wise, but would take more RAM.