Last year, ATI released only one card with two GPUs on one board - the 3870 X2 - which was the fastest card we'd ever seen. This year, though, ATI has released two further dual-GPU cards, based on both the HD 4850 and HD 4870.
The HD 4850 X2 has two cores clocked at 650MHz - slightly higher than the single 625MHz clock of the standard HD 4850 - with the remaining spec in line with the reference card. A 64-bit OS is pretty much essential, because of the 2GB of onboard GDDR3 RAM.
While the HD 4850 X2 was almost twice as fast as the single card in our most demanding Crysis benchmark - managing 30fps to the HD 4850's 17fps - it was actually slower at low settings. Call of Duty 4 and Call of Juarez results were reassuringly stratospheric, though.
Unfortunately, Far Cry 2 proved problematic: the HD 4850 X2 scored 58fps in our high-quality test - seven frames slower than the equivalent single card - and was only slightly quicker in less demanding tests.
This can be explained by fickle driver support. The 8.11 Catalyst driver refused to recognise the card - we had to use a beta version instead. We're hopeful that the 8.12 update will fix these issues, but can't be certain until we've tried it ourselves.
If you're willing to persevere then the $550card could become a "budget" alternative to the HD 4870 X2 - it's already faster, in some tests, than anything Nvidia has to offer. At the moment, though, it's just too unpredictable to make it worth the huge investment.