The only thing that is sort of bogging you down right now as far as upgrading your system and keeping it a viable option for upcoming products is the processor. This is simply because the socket is "sort of" outdated. There are still motherboards you can find for sub 100 dollars that have the socket 775, but I would recommend upgrading the CPU and motherboard both. If you are on a budget and just want to upgrade your motherboard (which I don't recommend just doing because it looks like you have a pretty graphics orientated setup, and you never know when the CPU will bottleneck the GPU), then this one, the ASRock G31M-S http://www.newegg.com/Product/Product.aspx?Item=N82E168... will probably work. It has a max support of 8 Gb of DDR2 RAM, Hyperthreading, and a fair number of rear I/O ports.
However, I would definitely recommend to upgrading to a more flexible system with the 1155 socket and the Z68 or Z77 series motherboards. The i-series processors that will run decently well with the powerful video card you have right now hover around $200. Visit newegg and see if you like any!