Get a flash cart thand can take SDHC cards. This means that you can put micro sd cards which are bigger that 4GB.
Old cards like the R4 can't take micro sd cards bigger that 4GB.
A few good (and cheap) flash carts are the Acekard2 and the DSTT. Good place to get them is
http://www.dealextreme.com/ . You can check the reviews there too.
I've had the Acekard2 for about 2-3 months and all roms that I've tried have worked with it.
Also with flash carts you can download "Moonshell" which will let you play music/movies/pictures on your DS.