A decoder card will use a pass though cable, only play DVD's and no other windows apps or games, will only display DVD's at 640x480, and will use up one of your card slots.
A scan convertor will also use a pass through cable. But that's where the similarities end.
You can display anything that your monitor sees, on the TV. Resolutions for DVD playback and gaming can be 1600x1200x120hz. No card slots required. Will work with any video card, so "WHEN" you switch out your Radeon for a better card, you can still use the scan convertor. This approach will save you money in the long run, as video cards will not need TV out. You'll already have a permanent solution!
Some good models are TView Gold, and Averkey by Avermedia, as well as many others, priced from $40 to $200 depending on features.