When you download Audacity I have found the beta to be relatively stable on my C2D iMac, but on my PowerBook G4 it seems to lock up more often. Stick with the last release if you are using it on a PowerPC computer.
It's easy. Just select MP3 as the format to convert to in iTunes's preferences, set the other options as you'd like, then choose Make MP3 Version from the Advanced menu after selecting the songs to convert. (Assuming you're using iTunes 8 - it's slightly different in previous versions of iTunes.)
Really, it's much easier in iTunes. Just CTRL-click the song and click "Create MP3 version" and vwhala! then delete the old version. itunes prefs you could change the bit rate of mP3 you would like it to be encoded to.
The main problem is i don't want to stay around my computer to do 1 by 1 and check the screen always. And i just know that some apps can convert wav to mp3 in bulk so that it can auto-convert.