Help with choosing optimal GPU for gaming at current rig

Hi all!
I'm a bit upgrading my PC, and now I'm choosing GPU for me (my rig: phenom ii x4 945, 2x1gb ddr2-800, antec 550w psu). As i won't uprade my RAM or go to new mobo with ddr3, I don't which graph. card and screen (1920* or 16xx*) to choose, because of bottlenecks.
Can anybody help me which screen and GPU to choose (max price for gpu is hd7850 level) and are there any bottlenecks from RAM to GPU?