The R4 is another nice card. Personally, I think it is one of the best options out there for playing emulation and backups of games you own. It is the size of a regular DS cart, and nothing goes into the GBA slot. The cart has a small slot for a miniSD card, and the r4 package comes with a USB device which you put the miniSD card into, and then put the entire thing into a USB port on your computer.
I recently came back from a vacation in Hong Kong, and almost everyone had one of these cards. If you haven't bought this Max Media dock yet, I highly recommend looking at the R4 card.