Why do my display drivers fail when i overclock?

i'm using the evga 8800gts, but when i overclock to a certain extent, my drivers fail and my display reboots, anyone know why? my gpu doesnt overheat, they just fail and reboot? any help is usful, ty. Also anyone know why overclocked i still lag in crysis with high settings and some on medium when poeple with the 8800gt get 40+ fps on all high with some very high. maybe its my ram ( 2 gigs, windows vista 64 bit) or my cpu, idk and help with that is aprectiated, ty. (please excuse the typos, im in a hurry :p)