I think this one is actually Archos's fault. There were rumors of an HD plug-in for the 5th gen players, and the TI chipset spec sheet claimed that it could handle 720p. But I think Archos got to testing it out with a variety of input file formats, and found it's playback insufficiently smooth. The HD plug-in was never released.
The new Archos Internet Media Tablets (generation 6) can do 720p HD playback IIRC. But so far as I have heard, no Dish Network plug-in is available for any gen 6 players.
Edit: Argh! I answered a question you didn't ask. I thought you wanted to transfer a Dish receiver's recorded HD program to your Archos via USB.
What are you trying to do, exactly? You want to plug your 705 into a DVR station, and then record the output of a Dish Network HD receiver using the component outputs?
This is not correct! SD mpeg-4 is their native video format. The flavor I use most often on my 404CC is divx. Not only will Archos players play back mpeg-4 videos, they can also record in this format.