a well encoded ogg at the same bitrate as an mp3 has greater sound quality. i have listened to the flaming lips cd as an aac (mp4) stream from their website (i'm assuming at 128kbps) and it was much better than either ogg or mp3.
you can encode music with the ogg codec right now, just search versiontracker for the utilities. you can also play back ogg through quicktime with the ogg component (also available on vt), which means that you can listen to them in the finder preview or in itunes with no modification.
if you have quicktime pro, it is possible to encode songs from cd to acc (mp4) but it's kind of a roundabout process (there is a shareware tool that makes it easier). it is expected that in the next update to itunes, you will be able to rip music into aac format.
as for converting your mp3s to ogg or acc, there is no point, they will remain at the same quality, you cannot make the quality better by putting it into a different format. if you buy a painting in a $5 frame and put it into a $100 frame, it's still the same painting. if you want your music to be of higher quality, you will have to re-rip it from the cd to aac or ogg.