An ATA-66 and even an ATA-133 drive will work on an ATA-66 controller. The issue is what size drive? Some ATA-66 controllers are limited to the 120GB drives while otehrs use 48bit addressing and can handle any capacity drive currently made (with room for plenty of growth).
I know the MDD FW800 model has 48bit addressing on both ATA contollers (It shipped with 180GB drives as one of the options.) I think the original MDD can also handle the larger drives on both controllers. Both MDD models have an ATA-66 as well as an ATA-100 controller for hard disks.
If your ATA-66 controller turns out not to support the larger drives, you can move you two current internal drives to the ATA-66 ccontroller and use the ATA-100 controller for the larger drive.