Yeah, you could do that... but I thought you wanted a new one with a lot of inputs and that's a big reason you wanted a new one.
I know you said you didn't need HDMI, but you could get a newer receiver with more features (like HDMI) for less than that new. I guess it really depends on what you're looking for.
About the upgradability of the 5805, I know they came out with a MKII version of it that had some upgrades. I'm not sure if you could get upgrades done to previous models to get upgrades like that though. If you did, it would surely involve going to an authorized denon service center to have it done.
A pre-amp would be like a receiver without the amp portion. The amps are what power the speakers.
The idea of your for just upgrading the video is kind of like one of the benefits to running separates. If you have a pre-amp / processor and then an amp to power the speakers, the amp should last you a long time. If new video and audio formats come out or new features, you just upgrade your pre-amp / processor and keep using your same amp.
If you got an AV-switch and a pre-amp, you would want to plug the switch into the pre-amp, but you would still need an amp to power the speakers.
I don't know if I mentioned this in the other thread yet, but I think a good general starting point for total system spending would be around 2:1:1 for speakers:receiver:sub. You should get a better balance in total final quality by spending the majority of your funds on your speakers. Your sub will also have a very large impact. The receiver will make a different too in quality, but I don't think it's anywhere near the kind of difference you would notice going with higher quality speakers.