AMD today launched their imaginatively named Radion 3870 X2, a week after their initial plan and a solid fortnight before I expected.
The GPU is billed with the codename R680, but in reality the name of the card tells it all: Its simply two of their R3870 chips on a single card with a PCI-E bridge and beefed up power circuitry, running Crossfire (AMD for SLI).
While multi-GPU cards have been done several times before (in reverse order, nVidia's 7950 GX2, The 3DFX Voodoo 5 series and, for those with long memories, ATi's ill fated Rage Fury MAXX cards) this seems to be a good implementation. The card itself is a single, 12 layer PCB (up from 8 layers on the 3870) rather than the stick-two-cards-together approach of the 7950 GX2, and the motherboard doens't need any Crossfire support. You plug it into the 16x PCI-E slot and it just works.
The chips are clocked slightly higher than the run of the mill 3870, at 825 rather than 775MHz, a paltry 6% faster that you most likey would never notice. The memory is slower binned cheaper stuff to keep costs down and runs at 1.8GHz, down from the 2.25Ghz of the single card, a 20% drop. From the look of the results a lot of games are more GPU speed rather than memory speed limited, as the performance is still good.
Speaking of performance, its not bad. It sits fairly consistantly between a single 8800 GTX and a pair of 8800 GTs in SLI. Which, coincidentally, is also where it sits on price, coming in at a relatively affordable $450. It does have a Crossfire connector, so when AMD get the bugs out of their Crossfire X drivers you should be able to add either another single 3870 or even another X2. To do so your motherboard would have to support Crossfire however, limiting you to AMD chipset boards.
The reason for the week's delay in launching was AMD wanting to get another driver set out. In their defence, this made a big difference, especially in Crysis where performance is pretty much good enough to play at a moderate resolution.
The good news Crysis-wise is that the 1.1 patch apparently gives you benefit from multi-GPU setups.
Given it will work on any mobo with a 16x PCI-E slot, the 3870 X2 is probably quite a good choice for upgrading. If I were to build a new machine I'd probably lean towards SLI however.
(edit) [H]ardOCP have also had a look and find performance to be around the same as the 8800GTX. Its worth noting that while they did use updated drivers, the results didn't come from the latest version.