realistically, how much performance can i squeeze out of a 640 with a 120mm zalman fan and arctic silver 5. i was thinking of going to 3.6ghz (an extra 400 mhz) but i was wondering if i could get it to 3.8 without sabotaging the life of the chip. i know these things can reach 4ghz on air "easy", but i don't want the thing to burn out in a year or two or become unstable. any suggestions? what kind of chips/clock speeds are you guys at?