http://kerneltrap.org/node/view/799
http://www.fearthecow.net/index.pl?s...st&page=kernel
This process is one PITA. If you feel up to spending some time go for it. There is so much that needs to be changed it's almost a waste of time.
As for the AGP issue, have you checked into the compatibility? You might check the changelogs for 2.6.x and see if anything had been changed for any SiS chipsets. Also, did you try the try_agp_unsupported option for agpgart?