Thatguy is completely correct here. The GX2 is NOT dual-GPU technology as advertised! It is simply two 8800GT cards with a heatsink slapped in between them. Also, unless you're planning on getting two, it won't be worth upgrading from any 8800 SLi setup, let alone any 8800 Ultra setup.
One more thing, the drivers are total crap right now. This video card is nowhere near what it can be, and it won't be that way until around mid-summer (my estimate). After the drivers are optimized and all of that, ATi's new high-end card will be coming out, and then the GX2 will be WORSE by comparison. Honestly, nVidia is kind of failing right now. It's far overpriced for what they're offering driver-wise, and also months (two to be exact) late.