I have my PII 965 set to 3.7ghz @ 1.4125v and its idle and load temps are roughly the same as stock speed.
Intel Burn Test 20 runs: 55C
I ran 20 runs of Intel Burn Test and it didn't crash. I hear people like to run Prime95 overnight or even longer, but I don't intend to do that. Heck, I don't even intend to ever have my computer on longer than 24 hours...
So, what are some other ways to test for stability?