When you say AMD is the more ideal tho, is this purely cost based or would the AMD have any other advantages over the Intel such as graphics performance say? The reason i ask is because i already have Intel processor from an old machine.
As long as your existing processor is based on Core2 architecture and has EIST/SpeedStep for power saving that's fine. If based on Pentium4-Netburst, too inefficient and valueless, you would be better off with a new processor.
e.g. I was in a similar situation with a spare E6300 Core2Duo. I happened to came across a XFX Geforce9300 mATX my friend was selling for cheap. The HTPC also runs some server tasks as well so it worked out for me.
The main reason to go with AMD (if not re-using your processor) is cost saving. For the same performance and features, plus a greater variety of motherboards to choose from that fits the HTPC criteria, CPU+Motherboard the AMD platform cost quite a bit less. There are also a few very cheap designated energy efficient AMD processors to choose from resulting in lower idle power consumption.