well, it comes down to preference really.
i have 3 pc's. an old pentium 2 i use for internet connection sharing, a athlon 3400 and a athlon 3800 dual core. the 3400 has a nvidia 6600gt, the 3800 has a ati 1650 pro. both the athlons run on vista ultimate.
my problems with nvidia :
1. nvidia menu does not function properly, alot of features are missing, greyd out etc .. )
2. TV-out full screen mirror function does not work, and nvidia has announced that this feature is dicontinued permanently. ( i use this ALOT. )
3. even the tv out itself sometimes resets for no reason when i reboot and i have to set the tv format back to pal again, it keeps jumping to ntsc.
4. fps is alot lower in vista than in xp.
5. nvidia are sloppy in delivering updates, they do 1 driver release in about 3 months.
6. i also HATE the new menu, it just sucks piles.
the list goes on ....
AMD/ATI
1. Menu works perfectly, with new 7.4 release all tv-out features that i need are working, even before 7.4 there were definitely far more features working than nvidia.
2. full screen theater mode works perfectly with certain players, i can't stress enough how important this is for me.
3. ATI has released 3 drivers in the last 2 months, and they have made noticable difference in performance and features in windows vista.
4. i have noticed that with each driver release fps has gone up alot in vista.
5. the ATI menu is just so much better than the nvidia one, atleast you have many more options to work with and it just feels so comfotable doing it.
in the end it really comes down to 2 things, i can handle nvidia's pethetic excuse for a menu if i must, but i cannot handle waiting around for 6 years for them to release new drivers, and most of all I NEEED FULLSCREEN VIDEO MIRROR FEATURE, i live on it, and nvidia has dicontinued this feature, and altho it only works with certain players on ati in vista, atleast it is working and they have not written it off. I am sure that ati does not see it as a priority at the moment, but they will get to it in due time, this is a comforting thought for me.
also have you tried their latest drivers ? it is a vast improvement over the last ones.
so in the end it comes down to preference, i prefer ati, it feels more stable to me, more features are working, drivers are less bugged. in the end ati gives me less headaches. but others might feel the same way about nvidia.
my point of the post above was that only little 12 year old children will go around and compare the top 2 cards and make 12 page threads about a 10fps difference when you really won't notice it with the avarage user, to me features and stability are more important than fps, unless the fps gap is huge.