Another thing I like to do is shutdown and restart the PC over and over. About 10 times is enough to stop a stability problem.
And then there is memtest86. http://www.memtest86.com. It runs from a floopy or CD so its great for determining if a stablity problem is hardware or sofware related.
But the real test is just to use the PC for a couple days to see if you have any problems.
3 tests insn't enough. You need to keep your CPU and memory usuage near 100% for a couple hours to see if it will start failing once heat starts to build.
If I can get the PC to reboot happily 10 times, I run Sandra 10 times. Then I run UD for two - three days, and just get on with games, apps and as many different types of program.
If it stays happy through all this, I dount it'll moan later on.