An moderate processor and maxed on your graphics card? That is the same like making an peanutbutter sandwich and let half of the peanutbutter fall of the sandwich
Putting up the best graphics card in the world together with an processor like an C2D is useless, as the graphics card is too fast for the cpu so you get an bottleneck.
Rather go for an phenomII x4 for example with an HD5870(wich is already an bottleneck) then an AthlonII x4 with the DH5970 for example, with the first one you will get better performance then with the second.