This mobo have the p35 that don’t support CF or SLI
So you cannot get a two 4890's but you can get a GTX295 or a 4870x2 and of course you have to OC you q6600.
What resolution do you use?
An other alternative is to wait for the new DX11 cards(comes out late this year) and upgrade at the same time to i7 + x58 + DDR3 + the new DX11 cards.