sharangad, you seem to be the expert in NVidia/H264 playback. I have a Geforce 6600GT (PCIe) on an AMD X2/3800+ (NVidia Detonator drivers 84.56), Win/XP Pro SP2.
In Nero Showtime 3.0.0.1,AVC-playback (in Showtime 3.0.0.1) appears to work properly. Turning on hardware-acceleration causes the info-readout to say "AVC (hardware accelerated)", versus plain "AVC". CPU-utilization decreases, though not as much as I hoped for. (I can play 1080p files, so I guess I can't really complain.)
On the other hand, if I try to play an MPEG-4 ASP file from Nero's Digital site (Greatest Game trailer "Cinema" and "AVC" profiles), hardware acceleration is broken.
With the options box "hardware acceleration" clicked ("enable"), the video-window is solid grey! The info-readout suspiciously shows "AVC (hardware accelerated)". I thought MPEG-4 ASP and AVC were different codecs?
If I unclick the Options box "hardware acceleration", then the video plays normally. The info-readout says "MPEG-4" (instead of AVC). Is this a bug in Showtime 3.0.0.1, or did I misconfigure something? CPU-utilization is roughly the same (actually, software-mode uses sligtly less CPU.)
Forgive me if this was already mentioned, but you can download more HD AVC trailers from
http://www.davestrailerpage.co.uk. (Most of these are direct, saveable links to Apple's hosted trailers.) Some of the more recent trailers have higher peak bitrates.