^ Don't be too scared of CM PSUs. My friend has a CM Elite 400W and it's working perfectly, I've got a Silent Pro M850 and it's pretty good too. Regardless, between the two I'd get the XFX anyway because it would probably be better quality. With one HD 5850/5870 a 500W PSU is fine, two HD 5850s you'll need a 650W PSU, two HD 5870s a 750W (recommended) and a 700W at least for an HD 5970. (Really, they're underclocked HD 5870s so they'd use less power)
The processor is OK, the Phenom II X4 955 should be enough, and if you want to overclock you will want a better CPU cooler. The RAM you have chosen is fine.
As for your overclock, there's no need unless you need more power.
If you live in North America, XFX is the best choice, otherwise it doesn't really matter - the Vapor-X models from Sapphire are nice.