Need help upgrading PowerMac G4

Hello everyone.. I am trying to upgrade my girlfriends old Mac.. It currently only has a 30 GB HD, and I was wanting to add some more memory and a larger HD. The problem is.. I am a PC user and I know absolutely nothing about MAC's.
I was looking at the Seagate Internal Hard Drive (160GB capacity, 7200RPM) But I want to make sure I get something that is compatible with her computer. I also wanted to add more memory but I have no idea what I can add to make her computer better without messing it up.

When I was trying to figure out what model computer she had.. I found a site that listed the mac with her characteristics as a Power Mac G4 (Digital Audio) with these possible model numbers... M7627 M7688 M7945 M7681. If anyone can help me and tell me what I can and cannot do I would greatly appreciate it.

Thank you in advance!

If it is indeed a PowerMac G4 DA, then it will only recognize up to a 120GB hard drive using the integrated on-board IDE. If you want anything larger you will have to purchase a discrete PCI/PCI-X IDE controller. Memory; the DA takes up to 1.5GB of PC133 SDRAM I do believe. Good luck,,