Good luck on being "future proof" for 4+ years. With any PC setup 2-3yrs is about as good as you're going to get, and with a mid-range setup you're going to get even less than that.
Bump up your videocard to a 6950 or a GTX 470 (you can go SLI if needed). I would have said GTX 570, but since you're doing HTPC double duty that particular card can't do bitstreaming.
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