Get 2 more GB of RAM, and maybe a faster processor. The 8800GTS 512 is still a good card, and should play most of the games you have fine at your resolution. If you were to upgrade it, I'd go for nothing more than a 4870 until you get a larger monitor. At 1280x1024, the 4870 is gratuitous overkill, if you're looking at bang-for-buck, try finding a 4770 in stock, or one of those cheap 512MB 4850s.
Your CPU is already overclocked, so you wouldn't see much difference going to a faster dual core (E8400/8500) unless you OC'd them quite high. Your best bet then would be either a Phenom II x4 940 or x3 720. The 940 is a quad core, which will help in rendering, but the 720 is AM3, is cheaper, and supports DDR3 RAM for future upgrades, should you choose to do that.