Upgrade to the Radeon 9800 Pro. It's better than the 6600 GT.
I'm curious to see this, the only I that I would think would give the 9800 an edge is its 256 bit memory, but I still don't think it can make that much of an impact.
They are both fairly competetive cards, but I think the 6600GT is the overall better card, especially considering the price.
edit - all right, it looks like my speculation was correct, but I still don't see how memory interface can make up for a 120mhz core speed difference.