I know I'm being nitpicky here, but if you're iMac has an Intel CPU then it's not the iMac G5. The iMac G5 used a PowerPC 970 (aka "G5") processor. Although the iMac Core Duo and iMac Core 2 Duo share the same cosmetic design of the iMac G5, that's pretty much where the similarities end.
QuickTime is an expandable application through plugins. Search for the formats on the web and you most likely will find a QuickTime plugin to handle it.