1: i m not sure since i m no expert, but i think that if the source doesnt produce DTS-HD then you cant hear it even if your AV supports it.
2: i dont think there are modern blu-ray players that dont upscale, but if it doesnt and you connect it to the A/V for video instead of plugging it directly to the HDTV it should be upscaled.
3: i had the impression that true HD sound was 7.1, then again i saw many blu-ray discs with a 5.1 true HD sound label... i m not certain as well.
about the PS3, since i have an xbox360 elite and its sooo noisy that i dont dare make it a source for a HDTV setup... how loud is the PS3 and can it reproduce divx, mkv etc? does it have HDMI 1.3 and optical spdif audio outputs?
If your Blu - Ray player streams the high def content via hdmi to the receiver the receiver would decode it. They do not both have to be able to -only one. You would want to decide whether you want the receiver or the player to do it and configure the player to stream it to the receiver or to decode it itself. They do not all do both some only decode and some only stream it to the receiver. If the player decodes then the receiver doesn't. The PS3 will decode it without the receiver needing to do it also but you may or may not need a firmware update. Yes you could get high def audio from a 5.1 setup and in fact in most situations that is presently all you need.
I would get a blu-ray that upscales regardless of the receivers capabilities. The upscaling on the receiver side is not going to be night and day, it will be a minor improvement. Most of the benefit from that is simplicity of setup.