ATI atm ;D
Well, I can't comment on any nVidia card other than my old GeForce 7800 GS. But I can comment on the ATi Radeon HD4870, which I also own, and I really can't recommend it for a Linux user. In Windows this card is a blast! But in Linux its a real disappointment (or better yet, I didn't want to believe that really not much has changed since I've switched to nVidia two or three years ago).
Installing the driver can be a pain in the neck- especially on new Kernels. For whatever reason, the driver ignores the xorg.conf (or parts of it) which is really annoying. And last but not least: I found this card absolutely useless for watching DVDs. For that I switch computers, using my old box with the nVidia GeForce 7800 GS.
Again, the card itself is a blast. But the driver for Linux is not. And personally, I don't think that this will change in the near future.
...and it replaced a Geforce 6600 (arguably the best supported card ever under Linux).
- installing drivers: indeed, fglrx used to be a pain. With Catalyst 8.8, not any more: it is PAINLESS. Even better, it can now create packages for your distro.
- ignoring xorg.conf: I found Nvidia's driver more at fault here, inventing modes that didn't exist and overriding my screen's EDID.
- app support in fglrx is medium to good: most work, and many are bug-free. Expect some bad behaviour in Wine though (checkerboard of doom) and more rarely, strange shaders gone bye-bye (Celestia 1.5 in OpenGL2.0 may behave strangely after a while). On well-working apps, performance and quality ranks up there (Nexuiz, all maxed out, does beautifully)
- free drivers make use of AtomBIOS; as such, support improves along most card families at the same time.
- a missing Xorg feature is vertical refresh buffer synchronization (vsync). Can be solved by using OpenGL as a video output and enabling OpenGL vsync in the driver.
If your main 3D use is Wine, go Nvidia.
If you expect to keep the card for a while and are ready to tinker, go AMD.
If you expect to keepa Windows partition around, go AMD.
3D on R600 and R700 in the open-source driver seems to be just around the corner (probably around the Fedora 10 timeframe).
why by a card that is defective by design?
I would buy the 4850.