Since it sounds like your getting dedicated graphics, there is no need to get a motherboard with onboard graphics (unless you want them). I recently got this Gigabyte motherboard and it works great for me: http://www.newegg.com/Product/Product.aspx?Item=N82E16813128392.
As far as video card, do you only plan on using this strictly as a HTPC, or will there be some gaming too, and at what resolution are we looking at? If it were just a HTPC, I would see no need for something more than a 5670. But for spending only 30 bucks more for a 5750, you will get a lot more card.
The upgradable future depends on when you want to up grade after you get this. Personally, I use a 3-4 year upgrade cycle, which seems to work out pretty well. In your case, I see no need to get a Crossfire setup. You would be better of just getting a new card.