Storage space is not different than closet space... You'll fill up all you can get! I've got a 4GB Sansa Clip+ that I added a 16GB microsd card to... It's full. Lots of music, plus all 7 Harry Potter, Atlas Shrugged, and about 8 other audio books!
Mark
Years ago they had a parody of a K-Tel commercial (young freepers, you can google “K-Tel”) offering “every record ever recorded” — Lithuanian Language records, Disco Polka”, etc. “And, if you act now, a free hairbrush.”
These days you can fit every record ever recorded in your pocket, for about the price of a hairbrush.
Other than having too many MP3 files, have you run into any problems with using the MicroSD card in the Sansa? I was just getting ready to do the same thing (my 4GB was long since filled!) and was curious if there were any issues with performance, or with downloading to the Sansa.
My Sansa MP3 player died in Iraq. I replaced it with a Sony device.