An AMD Athlon II dual core or an Intel i3 would be more than enough to support your needs. Even the lower generation of Intel's dual core is already enough. Unless you specifically require additional features such as low power consumption, speed preference, or anything else, the cheapest priced dual core CPU from both platform tiers would be more than enough. HD media playback is more GPU dependent then CPU. I'm not too familiar with nVidia's product, maybe the others can fill you in on this one.
Personally, my Athlon II X2 240 is a good candidate for the criteria. Sufficient amount of L2 cache, respectable 2.8GHz speed, and a fairly cheap price. But if you're looking for more raw power, get the Phenom or i5. They can handle your needs single handedly, or in this case, (maybe) single core-ly