This is quite the can of worms you've opened here.
The usual bit rate of MP3's is 128 kb/sec. This is comparable to listening to AM radio. They can be recorded at lower rates or at higher rates, lower=less quality/more songs per mb, higher bit rate=better quality/less songs per mb.
I personally have no idea what the little MP3 players will play as far as bitrate goes, but a good rule of thumb is that a minute of music on a cd takes about 10mb of storage on a hard drive. That roughly equals 1mb when encoded at 128kb/sec..... So a 256mb player should hold about 240 minutes of music... at a generic guess of 4 minutes per song you are looking at 60 AM radio sounding songs.
This information may be a moot point, as I have no experience whatsoever with little MP3 players... but I have over 52gb of music stored as 160kbs MP3's on an external hard drive..... that is slightly above FM radio quality, a compromise between storage space and old ears....
