1.) Not at all. Modern CPUs won't bottleneck the GPU at all. A 8x/8x board will hamper them slightly, but not enough to really matter. Last I saw, it was only a 4% decrease from the speeds of 16x/16x boards.
2.) Yes. 850W is definitely enough to handle them, especially with a high quality unit.
It's the speeds of the PCIe 2.0 slots. Normal speed is 16x, but when you add a second card, it might decrease. The board you picked is 8x/8x. It's generally not worth the extra $50-100 to get a true 16x/16x board.
Seeing as you haven't bought parts yet, you might want to post what you're looking at. I'm sure we can give you some better parts/prices. One thing I can say for sure is that you should get the X4 955 instead of the 965. The 965 is just a factory overclocked 955, so there isn't a real reason to pay more for it. Also, you should consider some 7xx chipset boards (the Gigabyte GA-790XTA-UD4 is excellent). 8xx chipsets cost more, but don't really give you much for the cost that you wouldn't get with any AM3 board. For example, the big draw is the ability to drop in an X6 CPU (which you won't need) without a BIOS update (which is free).
The motherboard you have selected it 8x/8x. The difference is that a motherboard has a set number of PCI-E lanes it can devote to the various card slots.
Most mainstream motherboards have about 24 lanes, which means that there is no way for them to devote 16 lanes to both graphics slots. If you buy a higher-end motherboard (for AMD the 890 FX chipset is top of the line), you get more lanes, something like 48+.
For your purposes you might see a slight decrease in performance, but as MadAdmiral says, probably not enough to notice.
Other than the board suggestion above, I really don't have any suggestions. The Gigabyte also has USB 3/SATA III support and 8 channel sound. In fact, most quality boards that I could recommend would have those features. Also, Gigabyte is just as high quality as Asus.
I should point out that to get the 955 to 965 stock speeds you can simply increase the CPU's multiplier by 1 in the BIOS. Given that the stock cooler is the same for both CPUs, you don't even need an aftermarket heatsink for the same speed. Save the $20.