Alot of people use programs like UD Agent, SETI, Folding@home and stuff like that to test stability. Another good one is Prime95. Since I'm on a superslow Mac at work so I don't want to look for the links, but just do a google search.
Sisoft Sandra is the best all around tester IMO, as it tests memory, hd, cpu, and all that good stuff, whereas the ones I mentioned only do the CPU.