SD = secure digital
You know those little memory cards for Mp3 players or video camcorders, or even palm pilots? (well about every new electronic gadget works with those now) its the same cards that you'll be able to use also on gamecube.
ranging from 8MB (sandisk will still manufacture them, not panasonic though) up to 1GB this year, and 4GB in a few years.
It depends how much money you want to spend, ranging from soon to be 25$ up to 1000$ and more.