Hello
I have done many upgrades and I would recommend going with Sonnet G4 upgrade CPUs. I have used both the Newer Tech 7448 and OWC CPU upgrades and they just are not as realiable as Sonnet Technologies.
Also I have used Sonnets SATA PCI adapter which worked great and I have used Sonnet's USB 2.0 PCI adapter. They both worked flawlessly and you never have to install any drivers either, both work with OS X.
*UPDATE*
Also you can upgrade the ram in a Digital Audio Power Mac G4 to 1.5gb with 3x 512mb PC133 SDRam. I would also do this, but be warned, you are going to spend about 125 ~ 150 to do this because of how old sdram is now. With that said though it is well worth the upgrade and it add much life to your machine.
Also When it comes to hard drives, you Mac by the design on the inside can hold at least 4 hard drives. What I did for a while is I had the Sonnet SATA PCI adapter is I had one SATA drives in my Power Mac G4 and two ATA drives.
Because of the address scheme on the Power Mac G4, your max ATA drive size is 120gb. So I had a 120gb and 60gb Seagate Barracudas ATA hard drives and a 320gb SATA Seagate Barracuda hard drive as my bootable drive. That gave me right at 500gb of storage space which at the time was excellent.
Theoretically if you got one of Sonnets 4 port SATA PCI cards (or two of there 2 port SATA PCI cards which is cheaper) you can four 1 terabyte drives up and running in your Power Mac G4 which will give you a whopping 4 terabytes of storage.
Also you can upgrade the CD drive to a Apple Super Drive that can burn at 20x and is 100% compatible with all the iLife software such as iTunes and iMovie from
http://www.macsales.com.
So yeah I would highly recommend upgrading a Power Mac G4 for two main reasons.
1. The Power Mac G4 is a rock solid machine, parts are readily available and the machine is a tank, well designed and works great.
2. Unless you are going to be doing serious video editing or image editing and design, there is no reason what so ever that an upgrade Power Mac G4 can't do what you want it to do. You will still be able to surf the web, listen to music, watch video, etc
Sorry that took so long I had not intended it to . . .
On a scale from 1 to 10, I would give it a 8 on upgradeability.
Hope this helps!!!
iGrant