Since you have the budget for it, you may as well go for an i7, it should speed up your video editing some. Mr Pizza is correct that you could get the same gaming performance out of an i5-760 build, though.
Of the 2 GPUs suggested, I like the Vapor-X, now that it's only $30 more (it was previously $100 more) than a standard 5870. I have seen articles suggesting that it's significantly quieter than a reference 5870. There is, of course, a combo/promo that ends today that may be of interest XFX 5870 + XFX 750W PSU - $480 - $40 promo code on the GPU - $20 MIR (if you want to deal with that). $440 for an 80+ Silver PSU and a 5870. $420 after the MIR.
I'd say that the Gigabyte UD3R is the cheapest fully-featured socket 1366 mobo. It should have everything you want...it's got USB 3.0, SATA 6.0 Gb/s, it can do multi-GPU setups...there's not really much more you can ask of it.
You can spend more on a mobo if you want, but it's not really necessary. I'd only get something more expensive if you like one of the additional features offered by a higher-priced mobo.
EDIT: HSF really depends on your budget. If you're planning on spending the same as the H50, you can get a good high-end air cooler for about the same price. Look at Frostytech's Top "5" Heatsinks list for ideas. The Noctua NH-D14 is top on many people's lists, but it does have a tendency to overhang RAM slots, potentially causing problems with RAM with tall heatspreaders (like the G.Skill or the Dominators). I have the Tuniq Tower 120 Extreme for my AMD build, as larger coolers wouldn't fit in my narrow full tower case.