from lspci -v: 01:00.0 VGA compatible controller: nVidia Corporation G71 [GeForce 7900 GTX] (rev a1) (prog-if 00 [VGA])
               Subsystem: nVidia Corporation Unknown device 0343
               Flags: bus master, fast devsel, latency 0, IRQ 16
               Memory at f8000000 (32-bit, non-prefetchable) [size=16M]
               Memory at d0000000 (64-bit, prefetchable) [size=256M]
               Memory at f9000000 (64-bit, non-prefetchable) [size=16M]
               I/O ports at ef00 [size=128]
               [virtual] Expansion ROM at fa000000 [disabled] [size=128K]
               Capabilities: <access denied>

from /proc/mtrr: reg00: base=0x00000000 (   0MB), size=2048MB: write-back, count=1
                 reg01: base=0x7ff00000 (2047MB), size=   1MB: write-through, count=1

command: echo "base=0xd0000000 size=0x100000000 type=write-combining" > /proc/mtrr

error message: bash: echo: write error: Invalid argument