Actually, your problem does not sound like memory, but hard drive space. By default, you lose 10% of your drive to the OS and formatting. To tell how much hard drive space you have go to:
>At the bottom of finder in the gray bar it will tell you how much space you have available on your hard drive.
Your hard drive should be no more than 85% full! If it is, delete, get a larger hard drive, remove items to an external drive.
GarageBand only needs 1.2 MB of space.
In order to use GarageBand, some additional content is required. If you do not already have the content installed, it must be downloaded.
Make more disk space available by moving some of your files to a different disk (such as an external hard disk or a USB flash drive), or by deleting files you don't need and then emptying the Trash