Bah.
I use both ATI and nVidia literally all the time. I play the latest games as they are released. I've never run into a problem on either brand that cannot be fixed with the latest drivers. Oddly enough, the last driver problem I ran into was an nVidia card on SC2, however, it went away with a simple driver upgrade.
I have more problems with Sound Blaster drivers than I do with video.
I don't believe in having a favorite computer brand. Every time I put one together, I do the research and buy what's putting out the best performance for the dollars that I have to spend. Over the years that's switched from Intel to AMD and back several times, and in the video arena, it's swapped even more often, and across many more chip manufacturers.
PS: I'm currently running an ATI card on Windows 7 64bit, and I haven't had a single problem since I installed it. Admittedly, I haven't had it long, (a few weeks? A couple months? Since a week or so after SC2 came out, anyway) and it replaced an nVidia card that also hadn't had many problems.