I do not want to use nvidia because there are no free 3d drivers for linux. On the contrary, AMD is cooperating in the development of the free x.org radeonhd driver.
Besides, I am not sure how much the GeForce 8800 GT 512MB outperforms the HD 3870 512MB. In the "Sum of FPS Benchmarks 1280x1024" with no anti-aliasing, the HD 3870 scores an average FPS of 539.50 whereas the 8800 GT scores an average of 564.20. And, the lowest price I find on newegg.com for the 8800 GT is $110. Not to mention the fact that it's using GDDR3 whereas the 3870 is using GDDR4. The FPS benchmark on Tom's Hardware doesn't specify which version is being tested, so in theory the 3870 could perform better than the test results show.
Anyway, like I said, I need the ATI for the free linux driver support. So back to my question. How much performance difference is there between the HD 3850 and the 3870 and does it justify an extra $30-$40?
I'm not sure if it has working linux drivers yet but you might want to check out the HD 4670. Usually performs between the 3850 and 3870 without AA but does better with AA. It also draws a reasonable bit less power than the 38x0 series cards. There are numerous reviews online if you google HD 4670. Newegg offers 3 models, a Sapphire with stock cooler and MSI with a non-stock cooler for $80 and an HIS with their IceQ cooler for $90 (but w/$10 MIR). At this moment the Sapphire is out of stock, but all 3 cards have been alternatingly in and out of stock since their release last week so I would expect them to have stock again in a few days.