I love my 260, but an 8800GT is still a pretty potent gaming card. I really do recommend the 2xx series of cards highly, they perform great. I have really enjoyed my card to the extent I have yet to find the max clocks on it because it's either folding or I'm gaming!
My G/F games on a heavily OC'd 9600GT that's been vmodded, it runs great but she's only running 1440x900 on a 19" widescreen lcd. I only game at 1680x1050, which for me is great for now, if I go any larger than my 22" monitor I'll have to get a larger desk lol!
Those e6400's are good processors, I saw in a post above someone recommended replacing it...if you do, don't get another dual...get a quad for a good price, even at that, not a big deal right now. But I would say if you could push a little more out of that 6400, open up your FSB a bit more that might help a little. At 3GHz though, that should be pretty sufficient for most games, there may be some games that are heavier on the CPU dependancy for emulated physics and such that might benefit from higher clocks, but I find 3-3.6GHz is a good sweet spot that is easily achievable.
4GB of DDR2 might help too, especially with more intense games. When that card needs to access textures from system memory and what-not, having more memory is definately a good idea. I'd definately recommend a decent 2x2GB kit, I've had great luck with both the G.skill 2x2GB 1000 and 1066, CL5, blue heatsink kits, both are great and very solid, very cool running and last I checked were around 50 bucks shipped on our side of the pond. Having memory capable of higher speeds might help with overclocking stability? I'm not sure of how that board OC's, but it seems to be pretty decent. Though 400-450FSB would be pretty good.
There's a few ways you could attack this, but I'd start with memory, reason being is on quite a few gaming rigs I upgraded to 4GB from 2GB whether OC'd or not, on XP, Vista and 7 all showed smoother gaming, faster loading, and overall an improved experience beyond just gaming. That 8800GT is a damn fine card, what kind of cooling do you have? Maybe you could consider improved cooling and a vmod if you haven't done so already? Again a lot of directions to go for sure, if Crysis is the only game giving you grief right now...for me that game wouldn't be worth upgrading a vid card for, but that's my opinion.
Though if you do get a GTX260, it will be a good experience I'm sure. They're great performers for the price, the drivers are pretty solid at least in my experience, and they are very overclockable.