diff -Nurp xserver-xorg-video-ati-6.12.99+git20090629.f39cafc5/src/radeon_dri.c xserver-xorg-video-ati-6.12.99+git20090629.f39cafc5-working/src/radeon_dri.c
--- xserver-xorg-video-ati-6.12.99+git20090629.f39cafc5/src/radeon_dri.c	2009-06-29 11:42:25.000000000 -0700
+++ xserver-xorg-video-ati-6.12.99+git20090629.f39cafc5-working/src/radeon_dri.c	2009-07-31 19:12:40.000000000 -0700
@@ -768,6 +768,9 @@ static radeon_agpmode_quirk radeon_agpmo
     /* Intel 82852/82855 host bridge / Mobility 9600/9700 Needs AGPMode 1 (deb #510208) */
     { PCI_VENDOR_INTEL,0x3580,  PCI_VENDOR_ATI,0x4e50,  0x10cf,0x127f,           1 },
 
+    /* SiS Host Bridge 655 / R420 [Radeon X800] Needs AGPMode 4 (lp #371296) */
+    { 0x1039,0x0655,            PCI_VENDOR_ATI,0x4a4b,  PCI_VENDOR_ATI,0x4422,   4 },
+
     /* ASRock K7VT4A+ AGP 8x / ATI Radeon 9250 AGP Needs AGPMode 4 (lp #133192) */
     { 0x1849,0x3189,            PCI_VENDOR_ATI,0x5960,  0x1787,0x5960,           4 },