Well last I heard the GX2 would be underclocked to 550mhz because of heat concerns. Do a search and you will probably find the links to the 9800 benchmarks and they are only marginally better than the 3870X2.
If you can deal with heat concerns two 8800GTs should be faster, but the GX2 will have a lot of overclocking headroom because they come clocked way below their potential. Just do a little research.
Rumor also has it that the 9800 could come out at the $600 price point held by the Ultra, but that doesnt make to much sense to me since the 3870x2 came out at $450 and is only only marginally slower.