Unlike your desktop, a higher resolution (i.e. 800 x 600 vs 640 x 480) displayed on your TV wont necessarily yield a more detailed picture. This is due entirely to the scan line (resolution) limitations of both the NTSC and PAL standards. Going for a TV output feature on a video card simply because it supports a higher resolution wont give you a better viewing experience.
On the other hand, while 640 x 480 is fine for your desktop and even games (it doesnt matter that much once youre running on a 35" screen), playing back DVD movies using a software decoder sometimes may be hurt by a 640 x 480 resolution limitation. A perfect example of this would be Zorans SoftDVD, which retains a performance hit when playing back in 640 x 480 vs 800 x 600, at the same time, some software DVD decoders may experience problems during playback at any resolution under 800 x 600. Translation? If youre planning to use your TV output for DVD playback on your TV, and you dont have a hardware DVD decoder card, 800 x 600 TV out support is more of a desired option.
The manner in which a TV puts a picture on the screen is not all that dissimilar from how a monitor does the same, unfortunately there is a dividing line that provides manufacturers with another gap to bridge, flicker. In the earlier days of TV output, obtaining a flicker free picture was not the easiest thing to accomplish, since then manufacturers have included workarounds for maintaining a fairly flicker free image on your TV, however the quality of these methods varies as well, this is the second factor youll want to consider when exploring TV output solutions on various graphics cards.
The clarity of text on your TV screen from a TV output on a video card is generally far from legible relative to what you see on your monitor. There are some manufacturers that have attempted to rectify this situation by offering software tweaks that supposedly increase the "sharpness" of displayed text, however the final decision is for your eyes only, making this the third factor to consider.