If you encode to MP3, do yourself a favor and rip/encode using something OTHER THAN PMM. I recommend EAC (Exact Audio Copy) and LAME. Since you have already ripped the music, you don't really have to worry about this issue. However, I don't recommnd using PMM to re-encode the files. There are plenty of good utilities available.
As to bitrate, using LAME's VBR (Variable Bit Rate) modes with Joint Stereo encoding is your best option. Fixed bitrate files (i.e. 192K) waste bandwidth on simple portions of the music, and can't properly encode the more complex portions. All songs have simple and complex portions - allowing the encoder to adaptively choose the bitrate allows for the best quality at a given size.
LAME's --alt-preset standard setting is the one most frequently used, although I prefer to give up a bit more disk space for --alt-preset extreme (produces file sizes equivalent to a fixed bitrate encoding at 230K).
Lots of people will advocate other formats (AAC, OGG, etc.), but in my mind MP3 is the universal format - it works on just about everything, so you never have to worry about it again.