You'll hear bad and good things about every brand. I'm partial to Seagate drives. They currently offer the awesome Raptor 10,000 rpm serial ATA drive that would rock with your G5. Seagate also makes the most quiet desktop drive on the market, the Barracuda. I've got a 120 gig 7200 rpm Barracuda and a 40 gig 5400 rpm Barracuda in my G4, they really are the quietest drives I've never heard
My dad's got Western Digital drives in his G4, never had any troubles. Not as quiet as mine though.