As far as CPU's go,
Phenom II X6's are pretty expensive last I checked. I'd go Deneb over Thuban. And yes Deneb and Thuban are plenty powerful for modern gaming. Really the only thing you will have problems with is Starcraft 2, Battlefield 4, and maybe Skyrim. My friend's rig has my old Deneb, 8gb of ram, and a used freebie 5870 from his bitcoin friend, I think the whole thing cost him around $250. He snagged a free power supply from another friend. Really a good computer. Plays Tribes Ascend and other modern games no problem.
Most 955 and 965's quads with unlocked multi can reach 3.8 to 4.0ghz easily which, given the lower price over Thuban, provides better value than the hexa core. With that said, if you get a chance to grab a Thuban for the same price as a Deneb, do it.
I think you will find benefit from an x3 at a lower clock to an x4 or x6 at higher clock.
Here is an example:
http://www.anandtech.com/bench/product/102?vs=83
If you can get one for around $60 and sell your x3 for $30 its worthwhile. If you have to spend about $100 for the CPU, don't bother.