I've followed the instructions from: 

http://dri.freedesktop.org/wiki/Building#head-1d3236b8fe603a3ecc41c7295609f538aca9b38c

and I get the following message when I attempt to load the mach64.ko module:

FATAL: Error inserting via (/lib/modules/2.6.20-r8/kernel/
drivers/char/drm/mach64.ko): Unknown symbol in module,
or unknown parameter (see dmesg)

There explains that it is a kernel-2.6.1 problem but I have 2.6.20 (gentoo r8). Here the relevant scanpci output:

pci bus 0x0000 cardnum 0x01 function 0x00: vendor 0x8086 device 0x7191
 Intel Corporation 440BX/ZX/DX - 82443BX/ZX/DX AGP bridge

pci bus 0x0001 cardnum 0x00 function 0x00: vendor 0x1002 device 0x4c4d
 ATI Technologies Inc Rage Mobility P/M AGP 2x