I'd imagine the difference would be getting shortchanged in the graphics department
why not try running the latest version of 3DMark from
madonion in the varous agp modes and see for yourself?
Check image quality and frames per second
Sorry did not know this was a follow-up to a previous post - I still think 3DMark would be a good way to check and I would undo all of th changes you made and try them one by one to narrow down the problem