1. The 990 will handle your current sources and display very comfortably, and sonically it's a remarkable piece of gear. There's an old comparison of the 990 to the 970 and the Yamaha 2500 (with the 2500 used as a pre-amp) done by a forum member that sonically put the 990 on top for both music and movies.
2. The three component inputs will support 1080i without any degradation (my HD cable goes through it at 1080i). I'm currently only running two component sources, but until just a couple months ago I had three (two at 480p and one at 1080i) and my picture quality was as good as running straight to my HDTV.
3. With HDMI v1.3 just beginning to emerge and the format war so confusing, this is an issue that leaves many people uncertain. The only sources that sonically benefit from HDMI v1.1+ are HD-DVD and Blu-ray because it supports the new audio formats (Dolby Digital Plus, Dolby TrueHD, and DTS-HD) that are only supported by those two formats. Even then, most players are offering multichannel analog outputs that work with the 990's 7.1 Direct input to provide those new formats with no degradation, plus many of the players have been re-encoding the new formats on the fly to high-bitrate DTS or Dolby Digital. Other sources (such cable/satellite and DVD) work just as well sending their audio over the familiar coaxial or optical digital audio connections. As for the video side of HDMI, the 990's DVI switching works with HDMI video just fine - as you may have read, HDMI gets its video structure from DVI so that the two are pin-compatible. Check out my
HDMI FAQ for more details.
Here's another thought: say you buy the 990 and an amp today, and then two years from now feel that you are ready to move to an HDMI v1.3 processor (both because you are starting to buy sources that make it worthwhile and because such processors will
exist at that point). You can keep your amp, sell the 990 on Audiogon for a good chunk of the original price (call it ~$600 or $700, although it's possible that it'd be higher), and use that to help pay for a new processor. This assumes that you'll be in a position to make use of HDMI v1.3 (or even HDMI v1.1) in the next two years, which would not be the case unless you bought a standalone HD-DVD or Blu-ray player.
4. Klipsch speakers tend to be pretty efficient (you can thank the horns for that), such that the 7075 could be a really good candidate for your case. Maybe some other Klipsch owners or 7075 owners could chime in here with more specific experience on the subject. At the very least, the 7075 would be a nice upgrade from the Harman Kardon's internal amp section.
5. I haven't put any sort of iPod dock on my 990 (just a stereo mini-plug to RCA cable hooked to the tape input, actually), but the DLO
HomeDock looks like it'd work just fine with the 990 - you could connect the composite or s-video and stereo analog audio outputs to one of the video inputs, and the 990 would transcode the video to the component output for you. One question that occurs to me (because I've heard of a few TV's being this way): does your HD-capable component input support 480i resolutions? If it does not, then the 990's transcoding wouldn't do you any good. It'd work fine with your three component sources, since all three support HD resolutions, but if this were the case for your TV you'd need to run an s-video cable from the 990's monitor output to the TV for the non-component sources.