Some of the:
- iMac's have a Model ID of "Powermac"
- some of the iBooks have a "Powerbook ID"
- early Mac-mini's have a "Powermac ID"
- eMac's also have a "Powermac" Model ID
...probably other examples as well. I know...it's VERY confusing.
I was made aware of these "anomalies" a couple months ago by another member.
So if the OP is correct...that they have an iMac with a Model ID of "Powermac 6,3"...it's probably some sort of G4 iMac model. Which in this case...no you cannot upgrade to Mac OS 10.6 because your computer does not have an Intel cpu (it has a G4 cpu).
HTH,
- Nick