I had good results with ffmpeg. It's fast and easy. Just like this:
ffmpeg -i realaudioinputfile.ra -ab 128 ac 2 -acodec mp3 mp3outputfile.mp3
add -vn to discard any video streams, assuming you want audio only.
I'm not sure, but you probably need to install the win32codecs as well.
And you'll have to use a separate id3 tagger to set the metadata strings of course. I like audio tag tool myself.
As for your second question, no, there probably won't be much, if any, savings, but that depends a lot on the bitrates you use. Also, as both formats use lossy compression, any conversion will result in a small amount of sound degradation. The copy-of-a-copy effect, if you will.