Missed your spec the first time, got it now.
There are no Vista drivers for the P4M800 North Bridge last I checked but Vista default drivers should work OK, specially after all the updates.
Do you have the
latest BIOS and drivers downloaded and applied?
Is this the driver you tried not sure if it is both for PCI-E and AGPl. Since the latest drivers are meant to fix and /or enhance PCI-E card's performance, you might have to use an older driver that works best with your setup, I had to try so many different OLDer drivers for my AGP HD3850 to get it working properly and still experienced issues with OpenGL in some games even on XP.
Who manufactured your 7300GT AGP 8X?
Is there an option do fully disable the on-board VGA and set the Primary adapter in the BIOS, apparently the on-board is not supported by Vista but there are some
third party drivers for it, wondering if it shows in the Device Manager even though you have an add-in card!
Did you have any issues while you were running XP?
Can you give us a reading from your power supply for the +3.3V, +5.0V and the +12.0V rails in Amps (max output) as well the make and model if you know.
How many optical drives?
Any other hard drive?
Any other add-in card /s.