
Activision has come forward with more information on how Guitar Hero's downloadable songs will operate on the Nintendo Wii. The good news is that the songs will be saved to SD Cards. So you won't have to worry if you're running low on internal memory. The bad news is that 200 blocks of internal memory is necessary to play the downloaded songs.
Karthik Bala of Vicarious Visions (the company contracted to create GH:WT) has this to say:
“Saving on SD cards is the way to go if you plan on downloading a lot of songs. The Wii system memory is then used as a “content cache.” This means you only need to have the free space for one song in the Wii system memory (about 200 blocks). … The game automatically transfers the song… plays the song, and deletes it when you’re done. … It all happens pretty quickly.”
200 blocks is the approximate size of an N64 or WiiWare title.

Guitar Hero: World Tour will feature two forms of downloadable recordings. The first type consists of master tracks of the industry's best known songs. Activision has not yet announced which songs will be available for download at launch.
The second form of downloadable songs will be user-created content known as "GHTunes". GHTunes are musical pieces that are created using GH:WT's built-in music studio. Activision has not yet confirmed if users will be able to exchange GHTunes across consoles.
Expect Guitar Hero: World Tour to reach store shelves October 26th.