You're correct on the headless part. Most of the time the iMac CRTs die and i'm surprised yours still works.

Yes, you can have a big drive but only to 128GB like War said and a 30GB drive on the same IDE channel. Thing is you wouldn't have some kind of a CD drive unless it was USB or firewire. Also you would need to make a new cable for the two drives since the IDE channel looks a bit SCSI like. Just need the 40pin connection, the rest is to power the CD-ROM drive.
To live is to let die