I think you should try some of these utilities to make sure nothing locks up and it's stable.
<A HREF="http://setiathome.ssl.berkeley.edu/download.html" target="_new">SETI@Home</A> - run for a couple of hours in the background, full tilt, while doing whatever you do with your computer. Maybe surf the internet, type, stuff like that.
<A HREF="http://www.tweakfiles.com/benchmark/3dmark2001.html" target="_new">3DMark 2001</A> - Just loop the benchmark a 4 or 5 times with an mp3 looping in the background as well. I know it says shareware, but you can still do the basic tests.
I'm not sure if SiSoft Sandra is any good. I don't use it, but it doesn't mean that it's a bad product.
Anyways, if your computer can run these without locking up, then you're all set...