That is entirely up to you. Since different people hear different ranges of sound and play thier music over different systems, their needs for MP3 sound quality vary greatly.
If you’re a sound quality buff, you’ll want to have MP3s encoded at a high bitrate (the floor for sound buffs is usually 192k, but many prefer 256 or even 320.) One thing to keep in mind is that several users at iPodLounge have reported problems with iPods playing 320k mp3s, a result of the iPod not having enough RAM to play the 320k properly.
If hard drive space is a concern, you may want to make your mp3s smaller – 160 or 128k.
The best way to tell what bitrate you’ll need is to do a test: rip the song in several bitrates and compare them to the original CD track. When you can’t hear a difference between the two, you’ve found your bitrate. Remember to test the mp3s on your iPod, not just on your computer.