On a system with an OC'd QX9770 (3.6 GHz), 4 GB of RAM, and a 4870 will have a FPS of about 35. Your PC is not of that caliber so I would say about 30 FPS average would be good. Here is a link
to Crysis Warhammer, which runs on the same engine, and a review with benchmarks. The PSU that comes with most PC cases is often not very good. You will also need a 64-bit OS to take advantage of all that RAM. In my honest opinion, I would hold off and wait until I could get that video card and a true quad-core processor.