Actually, I still think it's the RAM slots we're talking about... since there was talk about installing a 512 MB RAM chip.
The specs for that model of G4 states PC100 DIMM.
The PCI-port is a SCSI device (and as far as I know, no Mac's has used Ultra SCSI).
Apparently, (as far as I can tell from the specs) you can either install an ATA-harddrive or a SCSI harddrive (since the computer does have an internal SCSI chain). So depending a bit on what type of drive you purchased (the 80 gig) and what type is installed, there's different ways.
On a SCSI harddrive you set diffrent internal "addresses" (ranging from 0 to 7) and on an ATA drive you set the jumpers to master and slave. Usually there's a diagram also on the back of the harddrive (which you connect with the harddrive cable) how to set the jumpers. I think there's like 4 choices, master, slave, cable and something else (none or whatever, was ages ago I installed a harddrive in a stationary Mac).
Anyway... to know what manufacturer it is on your internal harddrive (if there's no diagram on that to tell you how to set master/slave or scsi address) you can find that out by going under the Apple menu "About this Mac". It'll give you different views depending on what version of Mac OS you use, but there should be a hardware specification stating what manufacturer it is that made the different drives installed (harddrive, CD, graphics card, etc).
Addition: the "About this Mac" should also state if the 10 GB harddrive is an ATA or a SCSI harddrive.
I know, all this technical mumbo jumbo is confusing, and there's even more mumbo jumbo, if the installation of the harddrive doesn't work right away. Some harddrives might also need to have the right speed set to them to work... oh well... maybe it's best to have an Apple techncian to do the installation (even if a regular PC techncian could do the work as well, just as he/she have the manual for the computer so he knows what buttons to push....