As I understand it, each video card is supposed to support some standard
output modes. When no custom video driver is available, the OS uses its own VESA
video driver, to make the video card work as a frame buffer. (If that driver
did not exist, you would not have a video display during the stages of OS
installation.) You'd suspect that driver was being used, when colors are
stuck at 16 colors, and display resolution is 800x600 or 640x480
(i.e. a pretty low res).
There are two parts to drivers. There is the video card driver for the
card. But there is also the motherboard chipset AGP driver, which
declares the protocols it is supposed to support. On one of my Intel
boards, you could change the AGP slot, between PCI protocol, or full
AGP protocol, just by changing the driver used from the chipset drivers.
Some chipset AGP drivers, also include a control panel for AGP in the
OS, where you can set a couple things. Again, this is manufacturer
specific, and needs to be researched first. For Intel, the settings
would be in the BIOS, rather than being a poorly written app for
the OS later.
Some chipsets have problems with their AGP performance. The video card
manufacturers know this, and they have a "quirks" list in the video card
driver, such that they will not use AGP speed settings known to cause problems.
For example, if the AGP interface will not run properly at 4x, the driver
may choose to run at 1x. The ATI driver in particular, has "SMARTGart", which
overrides your BIOS AGP speed setting, and does it is own speed setting. This
may cause the ATI card display to flash briefly during POST. Once the
driver is happy with the speed it has determined (or the quirks have told it
to use), it will not try a higher speed until you use the SMARTGart control panel.
The first release of SMARTGart was a disaster (caused crashes), but after
three or four attempts to get it right, it finally worked respectably.
I am not aware of NVidia doing the same thing. it is possible the AGP
setting in the BIOS, is in control for NVidia.
And then, you need to trace down the particulars for your motherboard chipset,
to see if it had any issues.
Towards the end of the AGP era, the last chipsets made finally had AGP
electrical interfaces, that properly implemented signaling. In the middle
of the AGP era, some chipset makers struggled to get their AGP slots
to run fast enough. And the marginal operation is what annoyed a lot of
users. If you use a board like the K7S5A, you would want to Google around,
to see if the chipset ever caused a problem or not. I do not see any
mention of a problem here, but perhaps there is a better guide somewhere