Heya,
In short, no.
In length:
Your resolution is what is keeping you from gaining any real advantage by getting something faster than the 8800GT. Your CPU will be the bottleneck on a newer GPU at the resolution of 1440x900. You basically won't see a significant difference for the expense of this upgrade. If you were playing at 1920x1080 with AA enabled, then yes, the GTX260 would show an improvement in newer games. At this point you would be better off upgrading your display to something much larger with a larger capable resolution. Then worry about CPU/GPU. As it is, the 8800GT is powerful for that resolution; even for Crysis (and I've played Crysis & Warhead on an AMD5000+ with an 8800GT at 1650x1080, so 1440x900 is definitely playable at mainstream settings). Every other game will still bow down to the 8800GT at that resolution. Even today's new games. The 8800GT plays a lot of new games at 1920x1080 perfectly fine (Batman ArkAsylum, Prototype, DragonAgeOrigin, GTAIV, etc). You'll probably see more of an improvement from a new CPU (higher clock) and a mild overclock on your 8800GT. Then, get yourself a higher resolution display.
I'd spend the upgrade money on a nicer display first.
Very best,