Yes, you can pretty much throw any IDE drive in there, within the size limitation you mentioned. Before I decommissioned my old 6500, I had an 80gig drive that I had salvaged from a toasted TiVO that had been freecycled. I moved that drive into a B&W G3 that I got later.
As to how to clone the drive, there are several things you could do. Depending on what you have to work with, one or another method may be preferable. One is to use a backup utility which will split the files among, say, 4 or 5 CD-Rs (this assumes, of course, that you have a burner). After you install a new drive, you would just do a restore from the CDR set. The appeal of this method is that it forces you to create an archive snapshot of your files that you can store for a rainy day.
Another would be to copy the drive's contents onto a scsi-connected external drive, and then copy back from that one.
I'm sure others will chime in with their favorite methods.
