You can use a program called RealProducer plus to change the bitrate but its just waste of time to do that because the quality wont be good. I suggest you to use program like eovideo/winavi/Winmenc to convert rm/rmvb file to avi then you can use tcpmp to play it.
I highly recommend the winmenc becasue its free, fast and great in quality.
http://www.hkepc.com/bbs/viewthread.php?tid=346269
If you want to play rmvb with the ppc realplayer, you can change the rmvb extension to rm. The problem is that it won't play properly if the bitrate is too high (above 250).