Western Digital Caviar Black Editions are generally considered the fastest/best non-enthusiast 3.5" HDDs atm. The 1TB+ Seagate 7200.11 drives had some firmware issues but I am not sure about the 7200.12 drives since they are pretty new.
I still suggest the newer WD drives though. If you are only using it for storage you can go with a green edition and the blue edition is between black and green performance wise.
Do _not_ waste your money on a velociraptor (or the older raptor). You would be better off getting two WDC blacks, short-stroking them and setting them up in a RAID 0.
I might be wrong but I do not think that the G5 supports hardware RAID. You can do software RAID though. If you want them mirrored that would be RAID 1 IIRC.
*Edit Actually, do not go for the black editions, the G5 has completely horrible HDD cooling so one of the cooler greens or blues would be far better off.
I should point out that I am not anti-Seagate. In fact basically all of my modern HDDs are Seagate. In my PC I have a single platter 250GB 7200.10, 640GB 7200.11 and 500GB 7200.11. I have another 500GB 7200.11 in an external case and a 250GB 7200.10 PATA in my G4. Planning on getting two single platter 500GB 7200.12s and a WDC black 640GB for my PC then moving both 500GB 7200.11 drives into my G4. :P These Seagate drives have lasted longer than my previous Samsungs and the only failed Seagate I have is an ancient SCSI Barracuda.
Of course, SCSI (tehe) All I use are seagate Terabytes About RAID, i'm using the raid function from disk utilities to pair some 80's in my G4, Redundancy will give you a good back up feature but not as feature filled as time machine or something, but if you want combined space you might want to use concatenated options does that answer the original question
For me, reliability based on brand or model is not worth worrying about. I see them last years or die out of the box, regardless of make or type. Google did a study which confirmed this random failure pattern across 100,000 drives. I always say: "Get the one with the longest warranty and back up your data."