An HTPC at it's heart is a desktop that is just hooked up to a TV. Typically, it's more specialized for video storage and as a DVR. So it is very easy to use it as your main PC. That's what I'm doing now (minus the extra monitor).
It will take nothing special. I would stick in a new ATI HD 5xxx series GPU for the ease of setting up multiple monitors. Assuming you're not gaming, an HD 5670 or 5750 would be enough, and 5770 would be good as a light gaming card.