Sorry I don't know anything about converting. I am always disappointed with real media files. I use Media Player Classic for rmvb files and it has practically no features; no fast forward or slow motion, no contrast or color adjustments. Since you're interested in rmvb can you recommend a good free player with good functions?
I use and recommend SMPlayer. It's got all kinds of easy to use adjustments including contrast, brightness, color and gamma. It also has fast, slow, and jump (10sec., 1min., and 10min. in both directions - very useful) and zoom. It even plays vlcs and flv files. Unfortunately, it doesn't play rmvb files.
I generally avoid rmvb files like the plague, because they're usually crappy compared to a nice avi.
Sorry I can't help, but can you recommend a real player that's free and doesn't suck?