Regarding putting the parts together, Google/Youtube is your friend! Connecting the cables, etc. is pretty straightforward as long as you read the instructions properly. Only real thing you need to be careful about is putting the CPU fan on correctly, otherwise you might fry your CPU.
Pushing the i7 920 D0 to 4GHz is very easy, although obviously you could decide to settle for a lower speed if the temps/power consumption is too much.
I'm don't really know how good a graphics card you need for video editing because that's not really my thing, but I'm sure others would be able to enlighten you...