Now, for futureproofing. I don't know how that applies to HTPCs since they don't demand much. My friend is running his with a dual core 2.8 GHz AMD Athlon 64 X2 5400+ (yes, I know the spec since it was from my old build), a 4 year old processor and it gives him no hickups, not even with Flash (from Hulu Desktop client in Win7) or Silverlight (Netflix app in WMC).
You're talking about bang for the buck, but when it comes down to it, HTPCs are really about price. Given that, I built mine for $360 including drives. I could have easily built it for less but I don't mind spending just a little extra to add a little spec, even if it will probably never go used. Even though I linked a few products, I would never actually use them to build an HTPC, I just can't justify the costs for its intended use.