It's good as it is, you can get the new video card.
Than you for your advice, but asked this question on the asus forum and this is the answer
Your Mother board uses a Nvidia chipset and works with SLI, Not Crossfire.
That ATI card might work ok but from past experience I have seen issues with Nvidia chipsets not working well with ATI cards.
A better choice in my opinion, would be a Nvidia GeForce GTX 460, which is comparable to the ATI card you name.
Also using a Nvidia card will give you the option to add another card in the future using SLI, you can't do that with the ATI cards on this board.
I am non the wiser?