Latest working kernel version:
None

Distribution:
Gentoo

Hardware Environment:
MB is an (old) Asus P4S8X
# lspci
00:00.0 Host bridge: Silicon Integrated Systems [SiS] 645xx (rev 02)
00:01.0 PCI bridge: Silicon Integrated Systems [SiS] Virtual PCI-to-PCI bridge (AGP)
00:02.0 ISA bridge: Silicon Integrated Systems [SiS] SiS963 [MuTIOL Media IO] (rev 04)
00:02.1 SMBus: Silicon Integrated Systems [SiS] SiS961/2 SMBus Controller
00:02.5 IDE interface: Silicon Integrated Systems [SiS] 5513 [IDE]
00:03.0 USB Controller: Silicon Integrated Systems [SiS] USB 1.0 Controller (rev 0f)
00:03.1 USB Controller: Silicon Integrated Systems [SiS] USB 1.0 Controller (rev 0f)
00:03.2 USB Controller: Silicon Integrated Systems [SiS] USB 1.0 Controller (rev 0f)
00:03.3 USB Controller: Silicon Integrated Systems [SiS] USB 2.0 Controller
00:04.0 Ethernet controller: Silicon Integrated Systems [SiS] SiS900 PCI Fast Ethernet (rev 91)
00:0a.0 Multimedia audio controller: C-Media Electronics Inc CM8738 (rev 10)
01:00.0 VGA compatible controller: ATI Technologies Inc Radeon R300 ND [Radeon 9700 Pro]
01:00.1 Display controller: ATI Technologies Inc Radeon R300 [Radeon 9700 Pro] (Secondary)

Software Environment:
xorg-server-1.3.0.0

Problem Description:
I have a AGP8x capable Motherboard and an AGP8x capable video card but AGP8x never worked for me. It says that bridge isn't capable.

A snip of dmesg:
agpgart: Found an AGP 3.0 compliant device at 0000:00:00.0.
agpgart: X requested AGPx8 but bridge not capable.
agpgart: Putting AGP V3 device at 0000:00:00.0 into 4x mode
agpgart: SiS delay workaround: giving bridge time to recover.
agpgart: Putting AGP V3 device at 0000:01:00.0 into 4x mode
[drm] Setting GART location based on new memory map
[drm] Loading R300 Microcode
[drm] writeback test succeeded in 1 usecs

Thanks!