The problem is I can't really put a quantitative measure to those numbers. Is a couple hundred a big improvement in performance? Are AMD really that much slower, and if so do they offer any kind of other positives I'm not aware of?
Man, it took me forever to find the CPUs you were talking about!
6850: 1748
6750: 1488
AMD x2 6400: 1306
The best way to look at this is percentage: the 6850 will be 17% faster than the 6750, and the 6750 will be 14% faster than the AMD chip. The 6850 will be a third faster(34%) than the AMD chip. In any case, perusing the site shows that you're looking at the bottom end of the pack, considering that they have the X5472 on the top of the chart with a score of 9,776!
Now, consider the score like the horsepower of a car - more horsepower means more speed, but doubling the HP doesn't necessarily equal double the speed. In a computer, not only do you have CPU speed, you also have amount and speed of the RAM, HD seek and throughput rate, how much cruft your particular software enviroment has, etc...
CPU speed is still important, so I generally attribute it 50% of a computers performance - thus a 14% difference becomes a 7% difference in real world performance perceptable to a human. Humans can't really notice anything under 10% on average. So you'd probably notice the difference between the 6850 and the AMD, but not the difference between the 6750 and either the 6850 or the AMD. You'll easily notice the speed difference between your old CPU(857), but it won't be twice as fast. Especially if you keep the old HD, as newer larger capacity drives tend to be faster.
Also, on motherboards.... ECS, Asus, Abit, MSI..... what's the difference? Are they all roughly comparable? Any I should avoid?
I've generally had few problems with the boards, but then, I don't buy enough of them to really have a brand preference. More important would be the chipset and what you intend to do with it. If you intend on overclocking the CPU/Video Card/Memory you're going to want a different(more expensive) board.