Get the GTX 275. It performs on par with the 280, is easily overclockable, and is cheaper.
NVIDIA GTX 275 Review
Performance that comes close to the GTX 280 for the price of an overclocked 260 puts this card as the one to get if a single GPU card is in your future. If looking to upgrade from a GTX 260, you could save some money and go with the GTX 275 instead of the higher end 285. The price is right, the performance is right, how could you go wrong with the GTX 275? Easy, you can't!