Some of you may or may not have noticed that AGP has been modularized in the new stock and [community] kernels. This is to allow people to build different graphics drivers against the kernel, especially the ati drivers.
If, after installing these kernels, you notice slow performance and recieve an error from Xorg like this:
RADEON(0): [agp] AGP failed to initialize. Disabling the DRI.
Then most likely you have not loaded the correct modules, or have loaded them in the wrong order. Most of you will be used to always having them built into the kernel and never needed to load them before
agpgart must be loaded first, before any graphic driver e.g. radeon. It is also a good idea to load any card specific agp driver e.g. intel-agp before the graphics driver
MODULES=(agpgart intel-agp radeon ...)
This affects, but is not limited to people with/using the following cards/drivers: i810, radeon