If you're running Dual 9800GX2 video cards, your AMD 6000+ will probably reduce your ultimate performance. Those GPU's are more powerful than your CPU is.
EDIT: BTW, Quad Core is still a good idea, but it's better suited for future software, which will more and more so become multi-threaded. With your 780a motherboard however, you'll at least have room to upgrade to one of the new Phenom II chips when they are released I believe. So you're not completely left behind.