As indicated a couple week ago I bought a 6600GT (see
http://forums.sagetv.com/forums/showthread.php?t=11602) - only to find out that it might not be that easy to find a big enough psu for my HTPC - which is based on the EA65 from Aopen. I returned the video card and got a Radeon 9550 instead.
I put the card in - and about 20 minutes later I felt that I did the right thing. I don't even see any need to upgrade to the latest catalyst drivers. The drivers from the CD (6458) are just fine.
The Cyberlink decoder from ATI produces quite a bit of tearing. My old NVidia decoder does not.
I use a DVI/HDMI cable to connect to the Plasma. The ATI drivers correctly discovers brand and capabilities. So, all I had to do was set desktop resolution and a little bit of overscan adjustment in Sage.
Switching back and forth between the DCT6412 and my HTPC (using DVD Standard Play) while watching HBO, I can't see any significant differences which does not mean there's no room for improvement. Since I have been traveling a lot lately, I haven't even opened the Plasma manual yet, nor did I make any adjustments on the ATI side. Still, it's very much usuable now.
FYI: I'm not talking about HDTV recordings made by Sage, of course. For that, I use my DCT6412.