In both cases it could be a number of different possibilities. I would run the RAM tester to see if the RAM is ok.
I would also check temperatures in both machines, usually you can do this in BIOS. You would run for a while and then reboot, going into BIOS immediately and looking at temps. Athlons do tend to run hot and if you didn't get the heatsink/fan right or don't have good circulation in the cases, you could be overheating. Another way to test this is to run with the case open and a small desk fan blowing air into it.
It could be your BIOS settings, if they are set too high for either the RAM or the CPU. Probably not if they are set low.
It could be the video card overheating as well, if you put one of the new ones in they can get real hot. The fan test would work for that too, but you will have to do more to figure out if it is the CPU or the video.
It could be software settings. Have you streamlined the WinXP? You can go to BlackViper's web site to get tips on how to do this.
Check some of these things out and post back what you find, more things may need to be checked if these don't pan out...