Unlocked is definitely the way to go if you can. You can play with settings to your hearts content then.
I went 905e for an HTPC and I really wish I hadn't. I paid the same money I would have paid for an Athlon x4 with a much higer clock speed (the cache doesn't do that much), and at idle they both draw under 50 watts. If I'd have turned off a core on the x4 I probably could have reached 3.5-3.6 ish ghz on stock volts, lower than stock with only two cores active.
Theoretically if you're going for dual core Phenoms, the quad core Phenom should be more energy efficient. Lots of dual core phenoms had 80W TDP, and their quad core counterparts had 95W TDP. Dropping that back to a dual core should net you a more power efficient chip. It makes sense to me that way any ways as the dual core parts were all simply quads that didn't get binned as high. Finding a 95W TDP Phenom II is a bit of a challenge though, as they don't seem to have been super popular.
For got to add, for HTPC duty, most of what I've seen shows go for better single threaded performance. Man I wish I'd bought that Athlon II x4.