Ditto for the Composite out. I tried this a couple years ago using RCA and a cheapo Radio Shack Amp on the head end, travelling 70 feet. The results were not terrible, as far as signal fidelity. The better the amp and cable shielding, the better your chances, but that gets into more money, a LOT more money.
As for audio over long runs, I did it two different ways, over 70 feet. First, I tried a lineout signal to an analog amp, and it wasn't bad at all. Signal diminution skews to the high end, I'm told, and once I was told that I could swear that there was more bass in the music, so I pre-amped accordingly.
Eventually the knowledge that my signal was skewing got too much to take, and I went out and bought another digital reciever, and fed digi-coax over the 70 feet. The theory here is that the square-wave, serial nature of the digital signal will hold up better over long runs. Having spent a bundle on that idea I immediately announced that I could hear the improvement, although I was never forced to do a blindfod test, which is lucky, because there wasn't a big difference and I'm not sure I could tell which was which. YRMV.
If I was doing it again (which I'm not) and had a lot of extra money lying around (which I don't), I would do the HTPC, right under the TV. I would have enough storage on board for all my music, and then run cat 5 to synch occassionally with el serverino. Then I would simply use a wireles mouse on my coffee table or a net remote and be done with it. Simplicity may not seem important now, but it will once you start laying cable.
For a lot of money, you can feed KVM a long, long way. Video facilities do it, you can too. But the idea is so crazy in this context that even I didn't do it. Alright--I tried it, but came to my senses and took everything back.