Just like any competitive market the leader always changes. AMD beat the living shit out of Intel all through Pentium 2, 3, and 4.....for speed and stability. The tables have turned as of late especially with Intel's release of the quad-core
As far as a video card goes, it will vary on what you are buying. For $150 you are going to get the same bang for your buck whether you go with nvidia or ATI. You won't notice much of a difference in the leader until you get in to the really high end cards. I personally like Sapphire ATI cards, they seem to be extremely reliable and I have never had one crap out on me ever....my computer is on 24/7 and I never have any less than 2 chars logged in at the same time. One thing to take in to consideration is the temperature of your computer. If your computer room is cool, and your case stays cool ATI will run just fine and be much more quiet. Nvidia tends to have much louder fans but the cooling set up on most Nvidia cards is better than ATI.