NVidia's proprietary driver (see http://www.nvidia.com/object/linux.html) supports TV out with X at 800x600 and 640x480 resolutions and is easy to configure, thanks in part to the good documentation (README) it is supplied with. It has some useful features such as support for displaying different pictures on a TV and monitor simultaneously, and being able to configure the size of the picture. Subjectively, with the cheap examples of each card I have tried, I felt that the quality of NVidia's scaler was worse than ATI's, but the NVidia had better brightness and colour balance. The latter bears out what I once read in a Usenet post by someone who had measured the signal strength of some cards and found NVidia to be stronger than ATI.

I will not cover the details of setting it up, because that's covered very well in NVidia's README, but I will offer some tips and observations.

I found that using X with TV out disabled the console (this could be because I was using a framebuffer console instead of plain VGA), so if using the card connected to a monitor and TV simultaneously (a good idea if not networked to another PC because text is barely readable on the TV) the best way to configure it is to combine the instructions for setting up the TV with those for setting up multiple X screens on one card. Currently these are appendixes J and R respectively. If the TV is set up as the secondary display (Screen 1), you can make a video player use it by setting DISPLAY=:0.1. TwinView (Appendix I) might be a viable alternative, but it may be more tricky to ensure a video player uses the TV in full-screen mode unless it's the primary device.

When using MPlayer in full-screen mode I found that it reset the TVOverScan option if I used the SDL driver, but it was OK with Xv.

Playing an interlaced video looked OK in terms of shearing, but this could have been partially obscured by the general blurriness, and the playback was not very smooth, as if the frame rate had been adjusted rather crudely. XvMC may help here, but at the time of trying I could not get MPlayer to compile with that enabled; I also had to disable MPlayer's OpenGL support. Newer versions of MPlayer seem to be free of those problems, but I have not had an NVidia card connected to a TV since.