Going frm a 8800gt to a GTX260 isnt going to give a big improvement. A HD4890 would be a bit better.
Thing is the 8800gt is still quite a powerful gpu & unless your going to shell out big money for a something like a gtx285 or hd4870x2 your not going to get big gains in fps. You'll prob have to be benchmarking to notice the improvements.
Also your cpu might not have enough power in it to really get the best out a HD4890 !