well at those resolutions the GX2 can pull its own weight, its better than the 260 and matches the 280 in most tests, and its cheaper than the 280 and in some cases even cheaper than the 260 now.
As for the 260 I wouldn't recommend as a buy, I would go for the 4870 or 4850, the 260 GTX is turning in the 9800 GTX, why buy a 9800 GTX when you can get a 8800 GTS 512.
Anyways, its power consumption requires atleast 500 watts so I think your good.