I have used Nvidia from GeForce 2 to 6600GT and never had many issues with them.
I started using ATI with a 9800 Pro, then a few X800's, then X1950 Pro and finally the main gaming rig is running a HD4870. Having ATI providing service from Toronto was a big factor in my switching.
I have experienced minor driver issues with Nvidia and ATI over the years but again minor. The X1950 Pro was installed since it came out and just replaced a few weeks ago never had any issues with any game and ran great.
Myself, I would not purchase the HD 4870 x 2 because there will be a greater occurrence with driver issues since it had 2 gpu's on the card. I know of some that have had issues on 3870 x 2 for example and not on the single gpu models.
I think AMD has a winner with the 48xx series and we should reward them for thier inovation by purchasing an ATI card. Lord knows they need our business ...
Ref Linux support, I always use an old Nvidia card like a 5900XT when I play around with Linux. I hear the support from ATI is better now but I have not tried it.
I stopped using Nvidia because of a driver issue with G80 cards, XP and the Unreal 2 engine (Ut2004, RvS, SWAT4, all unplayable on certain maps, 1-2 fps) A year after these cards came out it still hadn't been fixed on XP. Then again I just went through 4 driver releases in Ati land where crossfire was completely broken on XP with 3870's (broke in 8.2, fixed in 8.6) You'll get bad drivers on both.