I have pretty much decided I will be getting something from Emo to run whichever speakers I purchase. But I haven't figured out what to look for regarding the pre/pro solution.
Video Quality
Is it really important to have a good scaling ability? Doesn't the screen do that anyway in order to match its native resolution?
Yes, the TV does it, but most TVs do not do it as well as possible. Of course, most receivers/preamps/processors are also less than ideal at this. How important it is to have good scaling in a separate unit depends upon how well your source scales (if at all), how well your TV does this, and how picky you are about defects in the picture.
Does HDMI make much of a difference over component?
Assuming both are implemented properly, and assuming all sources have the same capabilities with both (which most do not these days), there should be little or no noticeable difference in picture quality. However, most sources these days do not have the same processing applied to both types of outputs, and the trend is going toward HDMI for better performance. Unless I were extremely poor, I would not even consider a preamp/processor/receiver that did not have HDMI.
Also, HDMI is capable of carrying audio as well as video, so you may want it for this as well.
Reon/Realta/others...what is the best?
As jliedeka states, the implementation of the chip is at least as important as the chip itself. There are DVD players that use the exact same upconverting/processing chip, but perform quite differently due to poor implementation in one and good implementation in another. So, look for professional reviews where actual tests are done, and don't just buy based upon what chip is in the unit.
Sound Quality
With the PS3 and the new bluray players, don't they do all the audio decoding internally? If so, what does the receiver need to handle the signal properly? It seems like it shouldn't need anything special if it is already decoded.
No, they do NOT all do the decoding internally. Some do, and some do not. IF you are using a player with a built in decoder, your preamp/processor/receiver needs HDMI that is capable of dealing with multichannel PCM OR you need to use multichannel analog connection from the source to your preamp/processor/receiver. Obviously, if your source does not have a built in decoder, then you need an HDMI connection and the decoding capability in your preamp/processor/receiver. It will certainly not hurt anything to have this capability in your preamp/processor/receiver, and all of the really good and recent receivers have this.
Before you buy, you may want to read:
http://www.audioholics.com/education/display-formats-technology/understanding-difference-hdmi-versions/
Whatever you do, do not assume that something with an HDMI connector can do everything that HDMI is capable of doing. Make sure the unit you are planning on buying is capable of everything you want it to do before you part with your money.