If you want a new card, that will make you feel a difference, then get a GTX 260 core 216. This seems to be the best deal in videocards market at this moment. If you can find a 4870 1 gb that is cheaper then that GTX, then get it, but that is almost impossible. Nvidia coded the hell out of their drivers and now the "new" GTX pretty much beats down the rival 4870 1gb in almost every game. You could also get a 4870 512 mb, if you find it cheap. This was my upgrade and it worth it, but I'm using a lower resolution then you are and I feel that 512 mb are not enough, for all eye candy, at 1920X1080.
Every other card that is bellow these : from 9800 GTX+ / ATi 4850 and lower, it doesn't worth the trouble. They will offer just a marginal increase in performance over your current 8800 GT.