i am having question that which card is more overclockable ? as far i know i cant increase hd4890 (1.313)voltage to overclock it more but is gtx275 can be overvolted to higher volt to oc it more ?
between the two get the GTX 275 with your eyes closed. You will not regret it. I've used this card for a few months before shifitng to DX11 and the card performed gr8 in games @ 1920x1080 res.