Don't buy a drive larger than 120 GB for the iMac DV.
Only machines after The QuickSilver G4 can handle more than 120 GB
(its possible with older machines with proper software or a pci ata controller)
It you get a drive larger than 120 GB your machine won't se more than ca. 128 GB of it. Any ATA 33/66/100/133 drive will work in your computer.
(SATA or SCSI wont work)