With current git I'm getting the warning below no matter I set for GARTSize:

in RADEONProbeOutputModes
compiz (video) - Warn: No 8 bit GLX pixmap format, disabling YV12 image format
start time: Tue Dec 26 06:00:00 2006

*********************************WARN_ONCE*********************************
File r300_mem.c function r300_mem_alloc line 225
Ran out of GART memory (for 1048576)!
Please consider adjusting GARTSize option.
***************************************************************************
ProcXCloseDevice to close or not ?
SynapticsCtrl called.

Relevant portions of xorg.conf:

Section "Device"
        Identifier  "Videocard0"
        Driver      "radeon"
#       Option      "MonitorLayout" "NONE, CRT"
#       Option      "MonitorLayout" "AUTO, NONE"
#       Option      "MonitorLayout" "LVDS, CRT"
#       Option      "CRT2Hsync" "50-75"
#       Option      "CRT2VRefresh" "50-82"
        Option      "AGPMode" "4"
        Option      "AGPFastWrite" "yes"
        Option      "GARTSize 256
        Option      "ColorTiling" "on"
        Option      "DynamicClocks" "on"
        Option      "TVOutput" "PAL"
        Option      "IgnoreEDID" "true"
#       Option      "NoDRI"
EndSection

My card is ATI Mobility Radeon X700.

PS. As can be seen from the option, GARTSize must be NUM, "NUM" is not accepted. That is unintuitive as with all other options "NUM" is accepted.