An HTPC really requires very little in computer hardware, I've discovered. A low-end Intel Haswell Celeron cpu (~$50), with its built-in Intel HD graphics, paired with 2 gigs of RAM is ample to render 1080p using any free Linux distro, though I'm partial to XFCE desktops such as Linux Mint Xfce. Anything more than 2 gigs is overkill, pure and simple. I stripped my HTPCs of their 4 gig sticks and reduced them to 2 gigs and noticed absolutely no difference in performance.
Linux really shines as an HTPC, and I don't understand why anyone would want to use Windows for that purpose. There's no reason at all. Why pay the license fee? Why wait twice as long for startup? Why bother installing an anti-virus? Also, if you go the Windows route, you certainly will want 4 gigs as opposed to 2 gigs. Windows is just more expensive all the way around, and there really is no need for it when the computer is intended to be an HTPC.
I steer clear of any AMD chips nowadays for two big reasons. One, AMD is wasteful when it comes to energy efficiency. Their low-energy offerings are pathetic and simply aren't worth considering against an Intel Celeron. Two, AMD's ATI graphics are horrible on Linux. I've run into nothing but problems trying to get their Catalyst driver working in Linux, to the point where I say "Never again." I won't even buy AMD chips for a Windows system, and why? Because eventually that Windows computer may one day be converted to Linux, and I don't ever want to have to deal with installing Catalyst in Linux. I don't think Catalyst runs that great in Windows either. The only video graphics I am willing to consider are Intel and Nvidia.
An SSD isn't really required for an HTPC in any way, shape or form, although I rather like using SSD for the operating system, simply because it allows fast booting. I consider an SSD a luxury.
Everyone should have an HTPC, and nowadays cost is not a barrier. Used computer components are amazingly cheap these days. There's only one component I will never buy used, and that's a hard drive. I have learned from hard experience to always buy new with a lengthy warranty. Refurbished or used hard drives just seem to fail at an extremely high rate in my experience, and I avoid them now. I think the main reason anyone would sell a hard drive is that the seller has doubts about its longevity. Well, the seller knows best. If he's selling his drive, it must be on death's door. Word to the wise.