Yeah, if your current frame rates are sufficient, stick with your 6600 gt. If you really intend on running the latest games oh high settings, go for the 7800, or 7900gtx. I'd reccommend you just save up your money, until you can decide on something to spend it on. Heres a suggestion, you might want to spend that money on a digital camera. Having one is really neat, and its pretty handy incase you want to take pics on the fly. Plus, many of the cameras arent really too expensive, and you can get a 4 mp camera for about 100 bucks.
Rig -- Emachines T5026, 3.06ghz P4, 300W Bestec PSU, 160 sata hd, 512mb dual channel 3200 ram @ 3-3-5-8, ATI Radeon 9250 pci, dual layer 16x dvd+/-rw.