GTX 560 ti should be sufficient for now. Why not wait a littlle and see what comes on the market soon?
If you really want to upgrade now, then go for Radeon 7970 or GTX 670, those are the high-end Cards with best bang for the buck at the moment.
Yeah, that's also my worry. I call it the "apple effect"; every time I gather enough money to buy something nice and shiny, something even nicer and shiner comes out as the same price as whatever I just purchased. With my luck, I'll buy a 680, then some industrial break through will happen and direct x 999999 will be released.