Go 4850. I wish I'd known the series was coming out in June. I would have waited and not bought the 3870x2. The 4850 is as fast as a 3870x2, which is faster than two 3850's or the ASUS 3850x2. What I can't figure out is why they're still selling 3850x2 and 3870x2's at a premium when the 4850 is out at just under $200.
A surprise candidate for cheap gaming is the new 4670 for $79 (especially with AA and AF enabled at 1280x1024). In Crossfire it actually approaches the 4850 and 3870x2, which means it beats the 3850x2 or two 3850's in actual Crossfire.