I have a Gigabyte 9500GT that plays everything (MPEG that is). No experience with h.264 files. Current ATIs in the 4500 or 4600 series have, I believe, more stream processors than the 9000 series Nvidias. Someone on the forum said that was important. Also, a few years ago, the memory path width was important, with 128K being the minimum. I don't know if a 256K path would make a difference with a modern card. My impression is that most users seem to be satisfied with 9400 and above Nvidias and 4600 series ATIs. I think the tearing that Nvidias experienced previously is resolved with Vista/Win7 and the EVR renderer.
HTPC: E7500, Nvidia 9500GT, ATI Theater 650Pro/Firewire Network Recorder/Avermedia Duet A188, NPVR 3.1.1, Win7 Pro
Test: Asrock 890GM Pro 3, Athlon X2 255 , internal HD4290, ATI Theater 650Pro, NPVR 3.1.1, Win7 Home Premium
Backup: Athlon 3200, Nvidia 7600GT, Network Recorder, NPVR 2.5.5, Win7 Home Premium