Make sure they are SATA 1.5 Gb/s compatible...
Some WDs need a Jumper on pins 5&6 to slow them to G5 Sata Speeds.
Don't buy 5400 RPM drives for internal use. They often lose communication with the G5 controller once they spin down.
Western Digital Black or Hitachi 7K drives are generally your best bet.
Western Digital SATA 6 Gbps drives will need a jumper on pins 5 and 6.
Hitachi typically work without special preparation.