I don't know where you got the capacity numbers from, but here's what I know to be fact.
If you record at 44KHz, which is high resolution mode, you get a total of 24 track minutes with a 128M card. So a 3-minute song with 8 tracks (3x8) will fill up the card. A couple of other factors come in to play though. If some of your tracks do not last the whole duration of the song and you record only during that time duration (like if you have harmony vocals only during the chorus), these tracks are smaller in size, leaving you more track time. On the other hand, if you want to do your mix-down on the MR-8, you need 2x the length of your song track-minutes in order to mix-down to tracks 7&8. In addition, you need more track-minutes if you want to bounce to a stereo WAV file on the unit.
The unit will tell you how much time is left in record mode, and always remember the Delete Unused function to get rid of undo files, which eats up more minutes.
I find my 256M card (48 track-minutes) adequate for a typical song of 2-4 minutes in length. I do have to mix down some of the tracks because I run out of tracks. I think a 512M card would be great, but I don't know whether you can use one with the MR-8.
I noticed that some other recording units can record more minutes to memory cards than the MR-8. I believe the reason is that these other units store the music in formats other than WAV (e.g. MP3 or proprietary), and they are in general much more compact in size.
DC