Hey Lasse,
I own the 2010 Quadcore with 2,8 Ghz (the one you posted in the amazon link) and it never used more than 60% of the CPU even with large sessions, several dozen instances of VCC and a few VST instruments. I'm running everything native so it should be way more than enough for a ProTools HD based system. I still think the new ones will be double as fast (perhaps more) at the same price, but life's life.
The Quad has an additional 4 virtual cores so Logic, PT etc recognize eight cores and distribute the computing power.
Personally, I would ditch the 27" cinema display because it's goddamn expensive and the prices will most probably sink in the next year or two. With the saved money you could go for the 8core version or the 3,2 Quad (I somehow wish I'd bought that one) at least.