The Athlon II is based off the architecture in the Phenom II, while the Phenom II X2 is simply a Phenom II X4 with a few cores dissabled. Anyway, the 7850 is based off the over Phenom architecture so it doesn't overclock that well and it runs pretty hot. Still, it's pretty comparable to the E5200, so it's generally slower than the 6300 by a good margin.
The Athlon II 250 is a better choice. It's generally a little above the E6300 and is easy to overclock. Also, AMDs generally have lower platform costs and the motherboard is not as much of a limiting factor when it comes to overclocking.