Powercolor is fine. As peaks said 3850 is the best you can get in agp, but there outrageously expensive. You can get a HD4850 pci-e for $10 more than the HD3850 agp.
PowerColor makes a decent card, although they have the fan run at 100% on some models like my PCIe HD 3850. Since all you are doing is playing back HD video, both the 2600 XT and the HD 3850 have a roughly similar onboard video decoder (UVD/UVD+) and support HDMI + audio. I would go with the less-expensive 2600 XT in that case.