Amd cpu with nvidia?

Intel does have a performance advantage at the same clock speeds with their most recent processors (Sandy Bridge, Ivy Bridge, Haswell). If you work with media they definitely go-to parts.

However, if you are gaming, AMD processors are still quite useful for those purposes. In addition, you will spend less money on the processor itself, which can then be put into a high-end video card or motherboard (which since you want to use NVIDIA SLI would be wise purchases).

If your primary interest is to buy an Intel CPU, keep in mind that the 3930K will be superseded by similar 6-core Ivy Bridge processors from Intel in a few months, and I am sure we can expect new Socket 2011 motherboards to come out with those new CPUs. Since multi-core gaming is not really a reality at the moment except in specific cases, you may want to consider a Haswell-based system instead.

In short...the AMD CPU you were interested in is quite enough for gaming with an SLI system, and you can put the money from it to other high-end parts (I would personally invest in a very good motherboard in this situation).

If you want the absolute most powerful system you can get at the moment at stock speeds, go with a Intel Haswell based system; it will fit in that category but it will be somewhat expensive (though likely less expensive than the Sandy Bridge-E system you were interested in).