Newegg had an evga 790i model, but it's already deactivated, and I didn't find it on pricewatch either. You might find it on ebay. If I were you, I would sell off your old stuff at anandtech or craigslist and upgrade to the i5 750 with a p55 chipset motherboard. Newegg has an evga model for only $75 after rebate: EVGA P55v 120 LF E651 TR LGA
There's no use in buying another 775 mobo just for the OC. If there's any bottleneck in there it's in games that run better on quads so even a big OC won't help you much. Add some RAM, you have a good system, next year or so you can upgrade.
A GTX 460 is faster but not that much faster than a GTX 260 OC. You may consider a HD 5850 or something.
Well your rig looks fine for playing games at 1680x1050, so if you can't upgrade to i7 now,then add 4GB RAM especially if you have a 64bit OS.
There are many games/apps which benefit from more than 4GB.