I have probably used more video cards than the lot of you together. I won't get into the details of how CompUSA's T.A.P. program works, just trust me like you'd trust the Lord Jesus and buy your next video card from them, and tell them you want the T.A.P. with it. (And don't worry later about thanking me, just spread the good word.)Anyway, being somewhat of a taste-tester if you will, I have had the oppertunity to see the view from both sides, up to and including the current lines. Here is what I have learned from all my travels.(cards I've had the pleasure and displeasure of using: Radeons 9800 128MB x2,9800Pro 256,and X800Pro; Geforces 4600 TI 256MBx2, and 6800 GT.)
ATI- ATI cards will provide better framerates while using enhancements such as AA and AF, to quite a staggering degree. The visual quality is also slightly better while using said enhancments, but trust me, it is all but unnoticable. Driver releases tend to not do much of anything.
Nvidia- Every Nvidia card I have used(one I returned to see if it was a defect), which has been in two completely different systems, has had an antistropic filtering "problem" in some games, namely Star Wars Galaxies,EQ2,and half life for instance, where it renders the AF in sort of an area in front of you, at higher levels, it is very noticable. One big bonus that is very underrated by benchmarks is Nvidia's Quincunx antialising, which mimics 4xAA at 2xAA speed. This is a very VERY important feature. When they say at 2xAA speed they are not yanking your balls around, and the difference between quincunx and 4xAA is a very slight blurring around very jagged edges, but trust me, it looks great. The other reason to buy Nvidia, and particularly the 6800 GT, is extreme overclocking with no mods. I have only used Coolbits registry edit to overclock, no hardware additons, and the 6800 GT I had was wonderful. It came stock with core/memory set to 350mhz/1.0ghz. With no anomalies in my test games (Doom3, NBALive 2005, NFSU2, EQ2,HL2) it could be set to 412mhz/1.12ghz. As a note, HL2 and Futuremark 2005 could both be run fully stable at 431mhz/1.17ghz!!!!
That being said, I will never by choice purchase an Nvidia card. For every comparable card I've owned, the ATI's have outperformed the Nvidia. Online benchmarks will support this. The one reason I'd go back to Nvidia, like I said is Quincunx AA. You won't find it on any benchmarks, but if you plan on running your games in 4xAA, compare the ATI's 4xAA to the comparable Nvidia's 2xAA, and it really does give the Nvidia card an advantage. Unless your playing some of the newer games that will not use AA unless it's enabled in the game, then your screwed(EQ2).
BTW, with the latest drivers, the 6800GT had no problems for me with stuttering.