Prime 95 is an excellent stability tester, but you'll also want to use something like toast or CPUBurn to make sure that your max temp isn't going to be too high to cause problems.
My PC would run Prime 95 forever, but then it locked up about 1 hour into some intensive gaming. I duplicated the lockup with CPUBurn and went and got myself a more powerful CPU fan for my SK6. All is well now!