graphic issues after updating

System:
intel powered iMac4,1 pre-installed with 10.4.

Problem:
After upgrading to 10.5 via a fresh reformat I have encountered visual oddities effecting what seems to be all programs.
ie; scrolling in finder/Safari leaves strange artifacts, often diagonal, making it unreadable. Grabbing and moving windows around has similar effect on the outer regions (especially if you shake it around).

Steps Taken:
I have installed all system updates with no noticeable improvements.
Also, running under a Safe Boot seems not to have these issues.

Any ideas?

Thanks, Sam

Stupid question (but at least a bump), but the graphic update was installed correctly?

As near as I can tell, yes the graphic update was installed correctly. I downloaded it and allowed it to install and re-start when prompted, at which time it installed and restarted. It succeeded only in getting my hopes up.

A little more info:
-This particular iMac is one of, if not the first intel iMacs available.
-The original system discs are nowhere to be found (office disorganization).
-I attempted to revert using the 10.4 system discs from a model or two newer resulting in a message early in the installation telling me in it can't be done giving no explanation, only reboot options. Are system discs now specific to their corresponding machines?