Sadly, you not only need Mac OS X but a more powerful machine.
Garageband has some significant hardware requirements in addition to being a native OS X application. So merely upgrading your iMac G3 to OS X won't be enough.
If you choose to upgrade your iMac (some iMac models can be processor upgraded) you may be able to use iLife 2005. But for Garageband you need at least a 600 mhz G3, and that is SLOW when working with garageband,
Macintosh computer with a PowerPC G3 (400MHz or faster), G4, or G5 processor
600MHz G3 or faster required for GarageBand
G4 or G5 required for GarageBand software instruments and advanced photo editing in iPhoto 5
733MHz G4 or faster required for iDVD
1GHz G4 or faster required to use HD features of iMovie HD
256MB of physical RAM (512MB recommended)
512MB RAM required to use HD features of iMovie HD
Mac OS X v10.3.4 or later
Mac OS X v10.3.6 or later required to use HD features of iMovie HD
Mac OS X v10.3.6 or later required to work with RAW photos (iPhoto 5)
Display with at least 1024 x 768 pixel resolution
QuickTime 6.5.2 (included)
DVD drive required to install GarageBand and iDVD
Apple SuperDrive (or 733MHz G4 processor) required for iDVD
RAW requires a supported RAW-compatible camera
Compatible SuperDrive required to burn DVDs
4.3GB of disk space required to install GarageBand, iTunes, iPhoto, iMovie and iDVD; or 250MB to install iTunes, iPhoto and iMovie only
iPhoto books and prints are available in the US, Canada, Japan, and select European countries
Streaming music via AirTunes requires an AirPort Extreme Card, AirPort Express Base Station (sold separately), and iTunes 4.6 or later
The iTunes Music Store is available only to persons above the age of 13 in the U.S., Canada, UK, France, Germany, and select EU countries