My first 3D graphics card I used about 6 years was a nVidia Riva 128 chip in a Diamond Viper 330 card. The drivers and updates were excellent, and a few years later nVidia were the leading brand in graphics chips, having squashed Voodoo chips (which I think got bought out by someone).
Since then ATI has come onto the market and have been playing catchup. In reviews I have read though, ATI has more than caught up, and has at times surpassed nVidia in its latest offerings (I saw a lot of reviews rating the Radeon 9800 as a better chip for performance).
Personally though, I still prefer nVidia, but consider ATI as a very very close second. nVidia driver support tips the balance in my opinion.