Okay, so this has been killing me and I haven't been able to find much help
elsewhere (ATI, HP, and Microsoft were all quite stubborn). Here is the issue:

I'm sure you're all aware of it. The Code 43 in device manager with several
ATI cards which are supposed to be supported. So, what gives? Judging from my
research, I've reached the following:

My AGP drivers are probably to blame, but guess who makes my chipset? ATI.
It's the RS300 (aka 9100 IGP) with a Mobility Radeon 9600. The card itself is
supported by ATI. The catch? They don't support the chipset. This is a
laptop, so everything came as a single unit. I call bulls---. Why? Misleading
information. They tell me that I can run Vista, but oops, my AGP drivers will
never work. When I contacted ATI, they told me to contact the maker of my
chipset, even though I said it was an ATI chipset. Do they not get it, or
what? I'm quite upset with the entire situation.

Will ATI put in the minor effort to support their own AGP chipset, or will I
be stuck with a brick if I choose to use Vista? I'm rather disgusted by their
poor approach to the situation. The best part of it all was getting linked to
a knowledge base article about nForce 3 being a problem, and how I should
contact nVidia. Idiots! Ugh. Thanks in advance if anyone knows a few
workarounds. I've tried most of the stuff floating around though...

And an update:
Well, according to some research, it seems that the issue may be related to
fast writes. I know that my card doesn't like to run without them enabled
(they are enabled by default and SmartGart always refused to disable them
back in XP). Does Vista handle AGP fast writes poorly? I think this may be to
blame. If that is the case, MS may be at fault, and would have to release a
patch.