A guy on a another forum has been saying that the HD4870 is basically as fast as HD3870X2 in games that scale well in Crossfire and faster in games that dont scale that well like Crysis.

Results he gave on the two most demanding games.

QX6850 @ 3.0ghz
4gb ddr3 1066
Vista Ultimate 64

Crysis Very High 1920 x 1200

GTX 280 34.2 fps, HD4870 26.9 fps

Lost Planet 1920 x 1200 4 x AA

GTX 280 38.7fps, HD4870 29.86 fps

Scores are good imo, especially if Crossfire scales well..