ATI's soon-to-be-launched dual slot, Radeon HD 4870, will score 12600 on a Q6600 based system. We are talking about default 1280x1024 3Dmark06 score.
Radeon HD 4850 scores around 11300 in the same benchmark at the same settings and if you get the resolution to 1920x1200 and use 8X FSAA the score drops to 7200 points. Radeon HD 4870 scores 9000 at these settings in 3Dmark06 and 7700 points in 3Dmark Vantage default settings.
Radeon 4850 scores 6400 in 3Dmark Vantage default settings and it drops to 3680 points when you crank up the resolution to 1920x1200 and turn the anisotropic on. Radeon 4870 scores close to 4600 points at these settings.
Overall, Radeon 4870 looks 10 to 20 percent faster than the GDDR3-based 4850 and you should even get playable scores at Crysis at 1920x1200, but of course No AA.