The BSOD is the first indicator you went too far. Back it off a notch.
The second is thermal run away. Keep track of your cpu temp as you increase the speed. It should go up about the same amount with each speed step. Say 1 deg C per 60 mhz. When you have gone too far it will start going up faster. That's time to back it off a notch.
When one of the above conditions are met (either one can occur first), run aquamark or sisoft sandra burn in with Cache & Memory, Mul;timedia and Math turned on. The temp. should stabilize about 3-5 degrees higher after about 10 minutes in both cases. Keep the max. temp. in gaming, benchmarking, etc under 55 deg C (lower if you can manage it without going deaf from fan noise). Also, if it BSOD's during burn in set it back a notch.
My system is great on temp. but BSODs right around XP3000 (2.16 gig).
Do not over clock video cards, they do not like it!