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.

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.