IDE drives top out at 750 gigs for no technical reason - the market just went to SATA.
Finding bootable SATA cards which just work isn't easy. It'd be a lot better to get a $15 USD SATA drive to IDE bus adapter and put it on a SATA drive. I'm doing this with one of my machines with a 1 terabyte SATA drive. Even in the older style G4 enclosure, there's still room for the adapter on the back. On the MDD, you'll have plenty of space. Plus, the IDE bus is pretty darned fast, faster than you'll typically get on a PCI card.
A MDD definitely supports drives larger than 128 gig.
Finding bootable SATA cards which just work isn't easy. It'd be a lot better to get a $15 USD SATA drive to IDE bus adapter and put it on a SATA drive. I'm doing this with one of my machines with a 1 terabyte SATA drive. Even in the older style G4 enclosure, there's still room for the adapter on the back. On the MDD, you'll have plenty of space. Plus, the IDE bus is pretty darned fast, faster than you'll typically get on a PCI card.
A MDD definitely supports drives larger than 128 gig.