I am sorry Asus linked me to a different board when I copied your board name in their site. Your board will not support SLI at all. The second slot at X4 speed stops that. Just upgrade to a single more powerful card and you have less problems.
Since you have an ndivia card now & your motherboard only supports crossfire, you would have to get TWO new AMD/ATI cards.
At a given price point, (cost of 2 cards = cost of more powerful single card), always go with the single, more powerful card. Less issues with heat & airflow, less complications, and if you need more power in the future, you can add another single card at that time.