On my machine where instability is absolutely unacceptable (for work reasons), I have my Celeron 533A overclocked to 880 MHz. This thing is stable 100% (with several months testing with heavy 3D apps and such) at 920 MHz, and I even have run the thing at 960 for a day (just for testing purposes) before it crashed. But 880 is a nice safe compromise and doesn't stress out the hard drives so much. (36.3 PCI - 110 FSB). Overclocking for family/friends is never a good idea, but if I ever sell this thing I think I'll just include the Alpha PEP66 and run it at 800 MHz. This 880 Celeron/Windows 2000 system is my MOST stable system ever, including several machines which were never overclocked.
If properly tested using good cooling, good components, and a good OS, an overclocked system will usually be MORE stable than typical department store off the shelf rigs. The issue is that people don't properly test their machines.
P.S. At 880, I'm getting 65% more OGR stubs done than if I were running it at stock. Now if only I could bring myself to erase UT. It's pathetic - I even have UT on my laptop.