umm choose a 650W Corsair one, it shouldnt come up with any problems if it does it will probably say you only require a 600W, but dont do it as they are lower quality (not bad quality) but you dont want a real expensive rig to blow up or anything, not that it would ever happen, im digging mysef a deep hole and someone will come along and say how they are fine, when i know they are fine but not the highest quality
or you could choose the 750W one incase you ever want to add more RAM, HDDs optical drives, fans etc, because it all eventually adds up
you could also put some of the money into a six core processor but that is optional futureproofing, getting a bigger PSU for second card would be better futureproofing anyway
thats ZOMG kitteh, search it on youtube (watch the raywilliamjohnson video - you'll probably end up watching his vids every week, i think he's hilarious!)