A few questions:
a) what is your budget?
b) I can't find a motherboard listed as "Asus P5P43". Is that perhaps a pre-built computer model?
c) what resolution do you game at?
Generally speaking, upgrading your graphics card will get you the best performance improvement. If your budget is around $200, i would look at getting a 5850. If you can go to between $250-300, either a 5870, a 6870, or a GTX 470. Upgrading to a lesser card (6850, GTX 460) probably is going to be underwhelming compared to the amount you spend on it. 5870s are coming down in cost right now with the release of the new 69xx series right around the corner.
Of course, if you game at 1200x1024, upgrading your graphics card isn't going to do much for you.