would the amount of GPU RAM help in converting movie files, or is it all CPU+RAM?
which particular card would you suggest for a little bit of video conversion and gaming in the 100~~USD range?
or would it not matter which card one bought?>
I've been wondering that as well. I think it depends on a lot of things, from the software you're using to the format itself. The test I ran on my PC with a Cuda-enabed 9800GTX and Adobe CS4, 64-bit with 6GB RAM and Core i7 @ 3.2GHz did almost the exact same as a lower quad-core with 2GB RAM on a 32-bit OS. I think the conversion I did cared more about processor speed than anything. But for other formats, you would do best to get a certain card that costs from $800-1200 (the CS4 NVidia card), assuming you have the money to get both the card and CS4.
SO, to answer your question, I think you would do best to just get a decent gaming card within your budget. Probably won't see much, if any, encoding performance gains.
All CPU+RAM. Movie playback is a CPU intensive task anyway; the only things the GPU does is display to screen, and improve image quality. Nothing is actually being rendered, so the GPU has no need to store a massive amount of information in its internal RAM.
I should point out, if the application in question is using something like CUDA that would add in GPU horsepower, it could be possible that there would be some minimal effect of extra GPU RAM, although I doubt that simply converting formats could ever eat up that much RAM at any given point in time.