Again, my 8800GTS 512 ran it fine. Here's a thread about comparison between that and 9800GT:
http://www.bjorn3d.com/forum/showthread.php?t=26464
What you can't gaurantee is that it'll work. In a normal PC it would no problem, but with those pre-built boxes there's always a chance it just won't. From what I can find about the motherboard, it seems likely it won't:
http://www.bjorn3d.com/forum/showthread.php?t=26464