Memory Cards:
This page will tell you tech specs and reviews for the various memory cards available for the DC.
VMU:
It's much more than a memory card. Now you can take parts of
the game with you wherever you go: download game characters to
the VMU and train them while on the bus; play standalone
VMU-specific games in the car; connect to another VMU to trade
files or compete against another person; share data between Sega
Dreamcast and Naomi-based arcade machines. And when you plug the
VMU into the controller, you have a private screen that cannot be
seen by your opponent. This screen can be controlled by Sega
Dreamcast and introduces a whole new strategic level of gameplay.
Features:
Hi-resolution LCD.
Built-in controller buttons and D-pad for standalone gameplay.
Display secret information during Dreamcast gameplay.
200 blocks of memory space for plenty of game-saving power.
Easy to save, trade, and copy save files.
Complete with cap to protect connector pins.
Nexus 4X:
This is the best memory card for your money. It does not have a screen like the VMU, but it has 4 times the amount of space. It also is connectable to your PC, so you have nearly infinite space. For those saving freaks (like me) you can even e-mail files to friends! Wohoo! Very handy for proving those high scores.
Features:
800 blocks of memory space for plenty of game-saving power.
Simple to save, trade, and copy save files. Just upload them to
the 'net!
Also comes with the neccesary connections to allow you to upload
your files to the 'net.
3rd Party:
This is the cheapest, and therfore worst memory card on the market. It has the same amount of space as the VMU (I think) but no screen. Only buy this if you abosolutely MUST have a memory card and can't wait to get a VMU or Nexus.
In conclusion, if you want more for your money, get the 4X. If you want to play VMU games and have a secret screen, get the VMU. If you don't want a screen and don't save very much, get a third party card.