ive installed my new 6600gt card, but its not working right. fs9 plays fine, but games such as doom 3, medal of honour etc, not so good....

im getting horrible lines, missing graphics, and black blocks of colour on the screen! ive tried the latest drivers, all the different settings, and everything else i can think of. ive a couple of ideas, but as im not too knowledgable on these things id love any hints you can give me
this card needs a power supply, which i have connected, but ive just read the booklet (again) and it says to use at least a 300 watt power supply, mines only 250 watt, plus its powering a sound blaster audigy platinum pro, as well as everything else. could this be the problem, the card isnt getting enough power? if so can i measure the output somehow? and how much power do these cards need? my only other idea is that maybe my cpu is too crappy, its a celeron 2.8 r and ive got 512 mb ram. sorry for the long post, but im feelin pretty frustrated. cheers guys, nick