My present computer is a dual core AMD running on an Asus M2NPV-VM
motherboard with this video card. "sudo lspci -v" reports:

00:05.0 VGA compatible controller: nVidia Corporation C51PV [GeForce 6150] (rev a2) (prog-if 00 [VGA controller])
Subsystem: ASUSTeK Computer Inc. A8N-VM CSM
Flags: bus master, 66MHz, fast devsel, latency 0, IRQ 16
Memory at fc000000 (32-bit, non-prefetchable) [size=16M]
Memory at d0000000 (64-bit, prefetchable) [size=256M]
Memory at fb000000 (64-bit, non-prefetchable) [size=16M]
[virtual] Expansion ROM at f0000000 [disabled] [size=128K]
Capabilities: [48] Power Management version 2
Capabilities: [50] MSI: Enable- Count=1/1 Maskable- 64bit+
Kernel driver in use: nouveau

with the nouveau driver running.
This is supposed to be the new standard in Debian squeeze with the nouveau driver replacing the old nv driver.
The problem is that with the nouveau driver installed .mpg .wmv .avi
and other similar files will not play with mplayer, vlc or aviplay. All
I see is either a pure green screen or a mostly green screen with parts
of the image in bars. The audio is fine. The flashplayer still works.
To disable the nouveau driver I have to turn off the kernel mode setting. I did this by creating a file I called 'nouveau-kms.conf'
with the single line:
options nouveau modeset=0
and installed the file in the /etc/modprobe.d folder. I next created an /etc/X11/xorg.conf file:

:~$ more /etc/X11/xorg.conf
# xorg.conf (X.Org X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg

Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "XkbRules" "xorg"
Option "XkbModel" "pc104"
Option "XkbLayout" "us"
EndSection

Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
EndSection

Section "Device"
Identifier "Configured Video Device"
Driver "nv"
Option "HWCursor" "off"
EndSection

Section "Monitor"
Identifier "Configured Monitor"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 75.0
Option "DPMS"
EndSection

Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1280x1024" "1024x768" "800x600"
EndSubSection
EndSection

[I added the "Option "HWCursor" "off"" line because I encountered
a problem with my mouse cursor disappearing both in nouveau and the
nv driver...]
After adding these files and running:
sudo update-initramfs -u
I was able to reboot with the nv video driver installed which works just fine. With this setting I can also run the nvidia driver from
nvidia: NVIDIA-Linux-x86_64-256.53 with a corresponding xorg.conf file.
The only problem is that with any kernel or xorg update I have to re-install the nvidia driver.
I guess my problem is a fix for the nouveau driver. I've filed a bug
report to Debian with no feedback. Looking through bug reports I can't
seem to find instances of this being reported & wonder if it something unique to my system.

Why don't you just use the nVidia-driver from the non-free repositories, instead the one from the website. This way the kernel-module will be automatically recompiled during update time, if necessary.