For rendering, you want the most stability you can get. If you're a professional, I'd lean toward a Xeon chip + ECC RAM and no overclock. Definitely an nVidia GPU such as the
560GTX 448 core which is pretty much on par with the 6950 2GB in gaming around 1080p resolutions.
If rendering is just a hobby, definitely get the 2600k (2700k if nearly as cheap?), either the 6950 or 560Ti 448 core will do just fine.
Bissa's suggestion for the motherboard is quite interesting.
-1200W is a LOT. Unless you plan on having 3 GTX580, you might consider something smaller.. 750-800 watt for crossfire/SLI, 600W for a single GPU is plenty.