Or if you know the cpu model number you can go to http://www.cpu-world.com/index.html All info on all cpu's. hope this helps SNO
You need to look up the max TDP for the specific CPUs in the systems you're considering.
There are different variants of CPUs that may have different power draws, so knowing that it's a Pentium 4 at 2.8Ghz doesn't provide enough information.
I suggest downloading CPUID and running it on each to determine EXACTLY which CPU you have in that system.
Cross-reference that on Chris Hare's CPU Electrical Specs site (Google it) for the Max TDP.
Use that information to determine what PSU choices would work for your hardware.
All the CPUs list the output for each rail. Your focus needs to be on the 12v rail.