Bigger and better is always coming. The new GTX 270 / 290's are rumored to be coming soon, but I haven't heard any hard dates yet.
Anyways, if I was going to spend around $300 on a graphics card today, my choice would be a GTX260 core 216 or a HD4870. If you fold, Nvidia hands down. In fact, even if you don't, the stock clocked GTX260 beats the ATI card in almost every test while the OC versions add some decent FPS on top of stock.
Check SKYMTL's review here:
BFG GTX 260 OCX MAXCORE (216SP) 896MB Video Card Review