I finally got around to installing a Sapphire HD 2600XT video card I had purchased over year ago in my HTPC! I delayed because the case I have is rather cramped and this card occupies nearly two slots and plus, I didn't want to break a working system.
Well I should have done it sooner. Once it was shoehorned in and installed I noticed I was now able to play HD content recorded OTA smoothly whereas before it was unplayable. The software I am using (GB-PVR www.gbpvr.com and PowerDVD8) both can take advantage of the GPU on this card and pass off signficant amounts of rendering work via DXVA to the card. So what was taking 100% CPU before and still not being able to play content smoothly, is now only loading up the CPU about 20% with completely smooth playback.
There were a few hurdles on the way. Firstly I had to update to the latest drivers for the card from Sapphire. Since this is a AGP card and the ATI 2600 chipset is PCI, Sapphire have written custome drivers to enable the card to work. Luckily they seem to be keeping up with ATI drivers with their latest drivers dated March 2009.
Secondly my HTPC is connected to my HDTV via HDMI. But the card also has an analogue VGA out which I also connect to my HDTV (so select the appropriate input on the TV to see analogue or digital). The problem I hadn't realised was, in this dual monitor configuration PowerDVD8 crashes on startup and GB-PVR goes into a CPU loop! So I had to disable the analogue monitor which I don't use anyway.
Finally there are still some driver problems. Playing HD content 720p 0r 1080i with hardware acceleration works fine. But when trying to play SD content such as OTA 576p material with hardware acceleration enabled causes both PowerDVD8 and GB-PVR to display garbled video with flashes of recognisable images. I have to disable HW acceleration to view 576p content which works fine with the CPU running at abtou 60%. The problem is, while I can disable HW acceleration in PowerDVD8 I don't appear to be able to do so in GB-PVR - it's on all the time. This means I cannot play SD content in GB-PVR which makes the setup less user friendly.
Still it's a major leap over what I had before which basically was no viewing capability at all.