A wonderful thing...You would have been better off with the 8800GTS with the extra memory. Your system should otherwise be fine.
You could go for the 8800GTX which eats anything at all, but is pricey. You could go down the sli route, but frankly I don't think you will gain that much.
Have you installed the latest drivers for the card?
Make sure you select your operating system from the menu displayed.