In my experience, buying MDs that are suitable for both SD and HD really limits you. I used to have all SD TVs in my house, so I built one MD for my lounge with a cheap and cheerful MSI mobo, AMD single core, 1 gig of RAM and a half-decent nvidia card, 7xxx if I remember correctly (obviously make sure it has an S-video output).
Then, when I was ready to upgrade to a HD tele for my lounge, I bought an atom 330 mini-ITX mobo (with nvidea graphics of course) with HDMI output at the same time, with the idea of moving my old MD to another room that still had an SD tele. I plan to go through my entire house in this fashion, recycling old MDs wherever possible. And when I reach a point where all the TVs in my house are all HD (some time away methinks!), I'll just sell the old MDs.
You might also want to consider using old laptops as MDs for your SD TVs, they can be picked up pretty cheap on ebay.
You didn't ask about the core - I assume you're sorted on that front?