OCZ and Geil are both PC overclocker RAM companies. OCZ has been around for a while, but Geil is a relatively new company tried to get a piece of the cake. With that said, I wouldn't buy RAM from them for Macs unless you can get it for cheaper than what Crucial's selling. There's really no reason why you should pay more for faster RAM when your Mac can't benefit from the faster timing. A lot of companies say their RAM is compatible with Macs, but what is certain is Apple uses Crucial/Micron RAM in their G5 systems.
You can get Crucial PC3200 1GB for $253, or a pair for $481, from Crucial.com.