what screen resolution are you playing at?
GTA4 is a very intensive game and a 3870, well is very weak. upgrading your graphics card should help out alot, then overclocking your Q9550 would be another large performance increase. probly get a 1gb version of a newer card over one with 512mb RAM will help out aswell.
With resolutions that low the GPU becomes less of a factor and your CPU speed becomes more important. perhaps look into overclocking to see if you gain any performance. the 4870/50s are good cards to consider.
edit: the 5770 is about as good as a 4870. the 5870 is as good as two 5770/4870 in crossfire. the 5850 as good as two 5750/4850 in crossfire.
also i should note that the 4870/5770 are slightly better than two 3870s in crossfire. just for reference.
edit again: messed up some of the #s