If you want a single card, it'll be either a GTX 260, GTX 280, or ATI 4850 or 4870
or maybe a 3870 X2, or 9800 GX2, if you count them as 'single cards' (a 3870 X2 has one PCB at least)
the ATI cards will be cheaper, and lower on power consumption; though there isn't enough information to say how well they'll perform against each other