I also started the groundwork to upgrade to wheezy from squeeze. I knew that I got a distorted screen under wheezy and it turns out to be my graphics card, which is a bit old, being a VGA compatible controller: nVidia Corporation NV11 [GeForce2 MX/MX 400] (rev a1) , as I learnt from 'lspci'. and it uses the nouveau driver under squeeze but not under wheezy. So I would need to - disable nouveau in /etc/modprobe.d/blacklist.conf and in /etc/modules.
Then to load nvidiafb i would need to create a file with this in it -
Section "Device" Identifier "my gpu" Driver "fbdev" EndSection#
then save it as "20-fbdev.conf" in /etc/x11/xorg.conf.d/ and I'll probably have to mkdir it.
It seems such a lot of hoops to jump through to get an unstable system where things do occasionally get broken, and to use it to replace a stable rock-solid system where stuff doesn't get broken unless I break it. So I've decided to stick with my current setup until wheezy becomes stable.