I didn't know that any plasma sets had DVI inputs but I can't see how ATi could consider that worse than analogue RGB via component! An analogue DB15 connector is better than component by virtue of its signal being closer to how film is encoded on a Telecine machine, with RGB and separate H and V sync, component syncs on green. You'd be hard pushed to tell the difference but if you have DVI on your plasma I'd use it.
The AGP 4x is a non-issue, especially when it comes to movies, you'll be fine.
Seeing as you won't be pushing the videocard too much you should be able to get away with spending very little. $50 or so on a Radeon 9200 should do the trick.
If you've got your money for nothing, who cares if the chicks are free!