Ideally, you'd mix twice- once for the CD, and another separate mix for the MP3 version. Both formats are different with regards to quality, bitrate etc. but also, they are both played on different things. Your CD version, is likely to have playtime in hi-fi's, home theatre systems, cars etc. but the mp3 is likely to only be played on low-fidelity equipment; iPods/mp3 players and computer speakers. Because of this, the approach to mixing the mp3 version is much different. You will want to mix on the kind of speakers that people will be listening on- computer speakers and ear-buds. Otherwise you'll end up with a mix that sounds good in the car, but lacking on myspace and on your iPod :) It is an art to get a mix that sounds good on both, but it's better to mix with the end media in mind :)
Don't convert to mp3 until at the end- listen to it critically when you've converted it, and see if the mix can be improved- or if there are compression artifacts, and adjust your mix accordingly :)
Edit: I'm not sure what the myspace audio upload interface is like, but if you can upload in WAV format, then do it instead of converting to mp3 and uploading there- you don't want to have to go:
WAV > MP3 > Upload (and convert again to myspace standard)
the extra stage of compression won't do any favours to the audio... but i'm not sure what options you have when uploading.
