I have a 6600GT in mine and got it working.
The only thing I noticed, is that with the nVidia drivers, if you EVER boot the HTPC without the TV activly connected (your input selector switched to the HTPC), it will never boot with the TV as a primary display ever again.
I actually have to reboot my HTPC with the TV on, then use VNC to turn back on TV output.
That was my big beef with nVidia.