Thank you for your question.
I can give you a general guide on how many songs you can fit on a given card size.
As you assume, the quality of the song or bit size will also determine
how many songs you can fit within a given space.
As a song is longer it takes more space. A 3 min 128kbits/s song will
take twice as much space as a 3 min 64bits/s song. You can of course
recompress your songs to a lower quality to fit more of them within a
45 minutes of music at 64bits/s take up 25mb, this would equate to
fifteen 3 minute songs. So on a 64mb card, this would be a little
under 40 songs. On a 256mb card, this would be about 200 songs and so
forth. However these are overestimates because songs are on average
longer than 3 minutes.
64kbits/s is what is considered as acceptable quality for codecs such
as WMA or OGG Vorbis. More likely you would be using MP3, and
128kbits/s is considered acceptable quality for MP3. At 128kbits/s
you're talking at approximately 1 megabyte for 1 minute of music. So
on a 256mb card you would fit about 70 songs or so. Apple quotes about
the same for its Ipod players. It lists the Ipod 10gb as being able to
hold 2500 songs at 128kbits/s, which would equate to 63 songs on
256mb. Or about 250 songs on a 1Gbyte card.
If you want to calculate how much you can fit within a given size the
formula is as follows:
size in mb x 1000 x 8 / (quality in bits/s x 60) = number of minutes
for that quality
256 x 1000 x 8 / (128 x 60) = 266 minutes
Multiplication is by 1000 rather than 1024 because the definition of
Mb in storage is not the same as the definition for data
communications or the one that software uses.
Personally I wouldn't recommend anything under 256mb, because then you
just end up having to swap around the songs very frequently, and on
long trips, the music gets repetitive.
I hope this answers your question. If you require any clarifications,
please do not hesitate to ask.
Apple Ipod specifications