i would do some kind of graphics benchmark between your 4870 and 8800GTS, and see if theres any difference in the benchmarks.
might sound stupid, but does the HDD have anything to loading/lagging?
like try defragging it, cleaning things up, closing background programs, etc.
is the game fully updated? Maybe try other games.
You should get a pretty high FPS, i get about 60-90FPS on multiplayer on max settings with my hardware and 1680 x 1050 res.
If your CPU is bottlenecking, does that mean that you will need an Quad Core to run it?