When it was time for me to buy a new graphics card I spent a lot of time looking around and reading benchmark reviews. The only Nvidia card that came close to the HD5850's performance was the 280GTX. But still the HD5850 was 10-15FPS faster and around £80-£90 cheaper before the prices got bumped up £40, just checked, they have dropped back down again.
Two HD5850's would cost me little over £30 more than a single stock GTX295, in addition I would get two free DX11 games (which come with each card) that I could sell on for £20-£30 each. Performance wise, game pending ofc, I've seen on average an additional 10-30FPS, in some cases even more performance. Also note that at the time, these benchmarks where done on beta drivers. The ATI 10.2 drivers are promising to support CrossFireX even better!
So, two HD5850's would give me better performance, 2 free games, DX11, built in multi-display support with eyefinity and in a month's time 3D multi-display support for an extra £30!
Now you know.