Heya Rory,
If you check out some reviews, you will see that the Radeon 3870 X2 is pretty much equivalent to the 8800 Ultra in performance. So you save 250 dollars or so and get the same performance.
Check out this review -
ATI Radeon HD 3870 X2 - Fastest Yet! | Tom's Hardware
You'll see in that review that in half the games it beats the 8800 Ultra. The 3870 X2 is by far the best bang per buck out there. It outperforms 2 standard 3870s in crossfire in most of the tests in that review.
At this second, you can't get any better than that card.