The fastest HD you can use on these machines is a PIO 3 standard, which is early ATA-2 drives. On the drive itself it should not list a speed higher than 11.1 mbps. or a cycle time of less than 180 NS. This should be located on the sticker on the drive.

http://docs.info.apple.com/article.html?artnum=16335

It looks like you need to hunt down an old PC to scrap.

Also, the OS limits your partitions to 2gb each.