I'll have to leave the squeezebox to others, but I can answer HDMI questions. it seems that 4 inputs is pretty standard in the newer receivers. Your HDTV will only take up the output, so no worries there, but you have to take stock of what you have/want to have hooked up to it. I have a PS3 and Xbox 360 which each have HDMI and a cord for my macbook, which I run directly to the TV since that HDMI doesn't carry any audio with it. most DVD players have HDMI these days, as all blu ray players and Cable boxes. I would go for 4 or so just for future-proofing.
The Wii does not support anything beyond component, so that won't take up an HDMI port, and most receivers these days will also convert analog to digital so that only one HDMI cable is necessary to run to the TV
