1) As far as I know, any DDR3 1600 will work.
2) if you aren't going to get into heavy overclocking, consider getting a 955. Both the 965 and 955 can be overclocked to ~3.6 GHz on the stock cooler. Consider the CoolerMaster if you plan a heavier overclock than that.
3) the 650 should handle 2x 5850s.
4) the difference between 8x/8x and 16x/16x is 2%. that means if you get 100 fps in 16x/16x, you get 98 in 8x/8x. Its not hugely significant
5) I'd pick the 300. the 900 is is just huge, the 300 is fine nd holds up to 6 120mm fans.
6) 30 GB should be enough to install the OS and a few (2-3) major applications but no games, get a bigger drive if you need more than that.
7) 6900s are out next month, no word on pricing, but 'more than the 6800-series' is almost certain.