"Well, a PSP takes USB and you can insert the expensive memory card in a media reader. You can stick an SD card, which is taken by the DS, in a media reader to backup saved games and things like that. The game cartridges for the DS are the memory stick itself."
The pro duo's aren't bad any more... as I have gotten my 4g for 79$. But Fry's Electronics has them for like 40$ every once in a while. 512mb for $10 at GameStop
It's so late(3a.m.), I've got nothing but those really long commercials.