My workhorse machine is a single core 2.2GHz 4 years old AMD! Sure, the 4 core and 2 core are faster when compressing DVD, but why should I do that on a daily basis?
Find the cheapest dual core with lowest power should be the way. I haven't see any dual core that can't play back HD movie so far.
Surprisingly, blu-ray DVD requires very small amount of data hence CPU power as well.
What else you'd like your PC do? Word? Email? Youtube. OMG, that HuLu needs 4 core!??