There's been a lot of good replies and opinions on this thread. I'll try to sum up the most important points.
1.) No matter which video card with TV-Out you pick up from which maker, you'll need to use Hardware Stretch for most games likely. This bothers some people more than others, you'll have to decide on your own. If you want to run all your games at their native resolutions without any stretch filtering / effects, you'll need either a good scan converter, a RGB connection from the video card straight to the TV, or an Arcade Monitor (again, using RGB).
2.) If you decide HW stretch is not so bad (or can live with the speed hit from the -sharp effect), then the quality of the TV outs vary. ATI historically has had great TV-Out for a lot of users on this board, NVidia's had mixed results. The Brooktree chip is not supported well by NVidia but other TV-Out chips might have better driver support. Poor driver support results in underscan (not filling the whole TV), the inability to adjust image position / size / etc.
3.) 2D speed and 3D speed are totally different. GeForces have great 3D speed but they're not the fastest in 2D. Different driver releases have an effect on the 2D speed, and it even affects rendering custom resolutions (which use no stretching effects whatsoever). I've heard Radeons are faster in that regard but I've never tested that myself. I wonder if there's any kind of solid metric which could be used to guage 2D performance...
Personally, I've used a bunch of Video Cards with TV outs and several scan converters. My own opinion (for my cabs at least) is to go with a scan converter that supports a wide scan range, this should let you use custom resolutions which is as good as it gets next to the whole ArcadeOS / AdvanceMAME setup (which is definately the most exact at the expense of ease of use). Of course, your mileage may vary...