Considering past memory cards for past systems may not have had all the memory one could have hoped for, Sega decided to change things up when they released the Dreamcast.  The Dreamcast VMU (Visual Memory Unit) is a step ahead in designing home system memory cards.  Included is a small LCD screen with working buttons to play mini-games along with other features (for example raising Chaos in Sonic Adventure or playing a Pac-Man type game with Namco Museum).  You can take the top off the VMU and transfer files into another VMU or link it up to play mini-games together.  The VMU can hold up to 200 blocks of memory, and while this may seem like a lot, most of Sega's 2K2 sports titles take just about 200 blocks of memory just to save options let alone a team season.  Not to mention that after a while the battery life of the VMU LCD screen will die, and that happens quicker than expected, but the LCD screen isn't necessarily needed if your just using the VMU as a memory card and nothing more.  All in all, I would recommend at least 3 or 4 VMU's for Dreamcast owners; they're cheap and affordable and you can still find them at game retailers.