If ya got some money to burn, there's other things you could 'upgrade' other than video performance (which I don't think you'll see much of a difference with your setup)
I use a
Filco Ninja Majestouch-2 Keyboard, which is kinda expensive but its the best keyboard I've used IMO and is very quite for being a mechanical keyboard. I would rather have this then 10 more frames per second on a game that you probably won't notice.
Then there's the
Gear Grip PC Strap and Monitor shield, which helps protect your stuff and makes it a lot easier to carry. Renegade has both of them in his shop.
But ya, I agree with the others here and you aren't going to see much of a difference if you upgrade your motherboard/video card.