I7950 or Sandybridge Core i7-2600K?

[Sp12]

Your assessment that SLI/XFire absolutely requires x16/x16 is off. It may not run at 100%, but 99% is pretty damn good and significantly cheaper.

Very much this. As of yet you can run at 4x/4x and still be fine with dual 480s (Hardocp had an article about it a while back).

 

[Sp12]

Nope. X4/X4 and it had basically no impact on the gaming experience. It was slower, but they didn't have to change any IQ settings. http://www.hardocp.com/article/2010/08/25/gtx_480_sli_pcie_bandwidth_perf_x16x16_vs_x4x4