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.
I've read that the 9800GX2 should come out around the $450-550 range. It's going to use 250 watts or more and will make INSANE amounts of heat. It will have two 65nm chips, and "potentially" have 2Gigs of memory.
So unless you have an ultra high res monitor, this card won't be worth the cost over something like the 3870x2 or a single 9800***. Just like the G80 Ultra is faster than the G80 GTX, but the Ultra isn't really worth the extra cost. By the time the 9800GX2 comes out the 3870x2 will probably drop in price and have much better drivers to boot.
Sorry that all my info comes from Fudzilla, but it seems to have the most collected information on the card and it also seems to be backed up by other sites as well.