My desktop system:
AMD Phenom 2 X4 965 3.40Ghz
4Gb Kingston HyperX DDR3
Radeon HD5870
My fps is about 22-25 average after initial drop.
What would be the best upgrade for me to up the fps? Is it very RAM dependent?
I only have 4Gb and memory is so cheap today so I might as well upgrade to 8Gb or more?
The performance has nothing to do with RAM, not unless you have a large model, and are running out of RAM so your system starts swapping.
Upgrade your CPU - in the end that is all that really matters.