GarageBand Learn to Play requires an Intel-based Mac with a dual-core processor or better.
Also, for good measure, if you want to take advantage of the AVCHD video in iMovie you must also be with-dual core Intel.
The baseline requirements for iLife in general are PowerPC G4 867MHz or faster with 512MB of RAM and 4GB of available Disk space.
Snow Leopard, the next major version of OSX, is widely believed to be an Intel only (ARM?) distribution as well.
You can buy iLife from Amazon, a 9to5mac affiliate, below.