I've had a single Visiontek 4870 for a week or so and all my games (COD4, STALKER, Alone in the Dark, Crysis, Rainbow Six Vegas 2, Gears of War, World of Conflict, and 3D Mark Vantage) run fine on it.
I just installed a second 4870 (Asus) by shutting down the computer, inserting it into open PCI-E x16 slot on my Rampage Formula, and connecting the two cards with two Crossfire strips. When I booted up, Catalyst Control Center couldn't detect the Crossfired cards, so I reinstalled it. When I did that, I could enable Crossfire and GPU-Z also showed it enabled.
However, apparently these cards can't be recognized by any programs. While they do appear working in Device Manager, the Aero theme for Vista is disabled and I can't run any of my games or 3D Mark Vantage. All the games say either the GPU is unsupported or insufficient. 3D Mark Vantage says I don't have a DirectX 10 capable graphic card.