Crocomire,
let me rephrase the question: what is the best prog to convert .rm files to a format that virt dub can use
Let me rephrase my answer: Try to use TINRA.
As many of us answered you, the best way is to search google for a prog that does the job. I suggested you TINRA, a program that converts RM files int AVI. VirtualDub doesn't support Streaming formats such as ASF (from Microsoft); somebody has released modified versions of VirtualDub to handles those kind of files, but not RM support, but supports AVI.
RM files, for the ones who don't know, is a video/audio file format by RealNetworks, mainly used for streaming videos on the web for his high compression ratio. RealNetworks used to give for free RealProducer, to let you convert your AVI/MOV files into RM to store them or even use on a server that does the stream; so it's easy to find rm files on the net such cartoons episodes (like SoutPark, and even tv shows, for example...).
Be careful, RM file use a lossy compression, so passing from RM to AVI (avi also compress data), you'll get a worst video quality.. If you plan to use no compression for your avi, you'll get HUGE files.
Hope this answer satisfy you and the other readers B)
Later,
Alessandro