If it is available in your area or online from your company of choice then go with the GTX580 to keep things simple and keep power down. The GTX580 will pay for it's self in power savings over having two GTX470 in sli over a few years use. Also 6GB isn't much any more when it comes to ram but for another year or two it will be enough for most tasks. Be sure to overclock that I7 for better fps.
1) sli GTX470 might be stronger, but I would generally pick the single GTX580.
It should be strong enough to handle any single monitor @ 1920 x ... resolution.
It also preserves your option to go stronger with sli.
If you are looking at 2560 x 1600, or triple monitors, then go the sli route.
2) I would pick the cheaper kit. There is minimal difference in real performance or FPS with faster ram or better latencies. Think 1-3%. Do not be seduced by synthetic benchmarks which are impressive, but not much meaningful.
I agree that the single card solution is far more preferable given the power consumption & heat output of a dual GTX470 setup, but 6gb of memory is plenty for gaming and nearly any other consumer use you can think of. The norm at this point is 4gb in most newer systems, and lots of folks are doing just fine with less. It really all depends on how many applications you are running at once. Even games that have native 64 bit support are only using a fraction of the memory available to them.
I would go for the single GTX580. It is a much more refined product and will offer plenty of performance, while being MUCH quieter and using MUCH less power.
For the RAM, there is a glaring difference between the two. The OCZ is at C6 latency, one of the fastest. The Mushkin is at C9 latency, one of the slowest. Get the OCZ Platinum.
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The two main factors for your RAM performance will be low latency and amount of RAM, not so much speed. 6GB of RAM is the minimum I would recommend for gaming with a Windows 7-64 bit setup. HOWEVER, if you are using Windows 32-bit, it will not recognize any amount over 4GB. Hopefully, you are using 64-bit, if not, only get 4GB.