If you were to Upgrade to ddr3 you just might be able to do it.
It in itself will give you a pretty good boost in speed.
What motherboard do you have right now?
If you don't mind, what are you planning on using more graphics cards for, 1 4890 is pretty good for most games.
You could save up your money for one enormous PC overhaul.
I found the mobo of my dreams MSI 785GTM-E45 Motherboard