It certainly isn't pretty, but the GX2 still packs a heck of a punch. Even now, with Nvidia's newer GTX 200 series upping the stakes, the old 9800 dual-GPU card impressively holds its own on both performance and price.
Essentially two 9800 GTX boards in one enclosure, it offers a hefty 256 stream processors and 1GB of RAM; it balances the huge amount of extra power by scaling back the core clock to 600MHz, making it very similar overall in its core specs to the 280. It requires the same six- and eight-pin power connectors, but the GX2 is the only way you'll get quad-SLI - by lining two of these bad boys up.
When it first arrived it may not have been the incredible success we hoped, but it has benefitted since from major improvements in Nvidia's drivers. In Crysis, at very high settings it managed 28fps, four more than the 280 - although not quite as fast as ATI's own dual-GPU HD 4870 X2, with 35fps, or the new kid on the block, the GTX 295, which top-scored at 40fps.
The GX2 managed an extra 15fps over the GTX 280 in Far Cry 2 at high settings, but still lost out to the 4870 X2 and GTX 295 cards by more than 30fps. In short, the GX2 is fast, provided you have a big enough monitor to really push it to its limit, but it can't beat the similarly priced ATI king-hitter for raw power.
It's around $170 cheaper than the HD 4870 X2, which might sway you, but if you're the type of person who's willing to spend a frankly ridiculous $572 on a graphics card you'd be mad not to pay the extra to get the best in the business. Unfortunately for Nvidia, the 9800 GX2 isn't quite it.