You can use TINRA:
http://guiguy.wminds.com/downloads/tinragui/You'll need a fast PC, and plenty of drive space if you're converting anything big as it saves the data uncompressed (i.e. - big)
Actually, if you use the GUI you can pipe the output through an Xvid codec or somesuch, YMMV.
Then if the AVI plays ok, just run it through PSPVideo9. If it doesn't, you'll need to run it through VirtualDub to correct sync and framerate issues. Complication varies depending on how the original .RM was encoded (RM can have any framerate you want so they're often not "proper" fps).