the introduction of Creative sound cards that have the 5.1 dolby digital decoder on the sound card itself,
No. SB Live 5.1 is just a 6 channels analog output sound card, the dvd software does the DD decoding. You really believe there is DD decoder in a $30 (used to be $60 for oem card) sound card?
how does the TV out on decoder cards compare to the TV out on Radeon's?
This is apple vs orange. The tv-out from vga card outputs everything to tv, the tv-out from the dvd decoder card only outputs the movie playback to tv.
Draw back of dvd decoder card:
1. If you don't use the pass-thru cable, you won't see the movie scene in games such as wing commander iv dvd version (this game is bundled in Creative's 1st gen dvd decoder card + dvd-rom drive package) on monitor.
2. Pass-thru cable degrades the display on monitor.
3. Take up 1 pci slot unless it's a daugther card attached to the vga card.
4. Need to use proprietary driver and software.
I've the 1st gen Realmagic Hollywood in the closet.