honostly, id try your best to stretch your budget a bit further so you can get a board that will support at least 2 x 8x pci-e slots, if your going to get another 7850 later. the ecs would be "alright", but that 2nd slot is 4x and it will cut performance down on the card thats in it, and will make crossfire pointless for the most part. i dont know if you prefer micro atx or standard atx, but this one looks real good spec wise for what you really want.
its only $20 more than the ecs board you linked. (well worth it)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813188081
its a great brand, 2 pci-e slots @ 8x, and if you ever decide to get a k series cpu down the road, you will be able to overclock it pretty well, and this will support ivybridge cpu's i believe, (you may need to flash the bios to a up to date version), so you will have that upgrade option as well.
the rest of your parts look good, but, again, for the sake of spending money wisly, you should try to stretch your budget a little for the 2500k, so you have more cpu power through overclocking, which will help out when you have your 2nd card, as well as be able to keep up longer with programs and what not in the future compared to the 2400 which will be stuck at 3.1Ghz forever. compared to the 2500k, it comes stock at 3.3Ghz, and can go up to 4.8Ghz if you ever need more power in the future. (for just $30 more than the 2400) again, well worth it.
for these 2 things, (evga board, and 2500k)..it will be an extra $50-$60. if you dont have it, it shouldnt take too long to get it, then you will have a solid system that you will be very happy with. you asked how much of an upgrade this will be compared to what you have now? well, it will be a HELL of an upgrade over the q6600 you have, should open up your cards power more, and will deffinatly be key when you get your 2nd card in crossfire. get the 2500k, overclock it to at least 4Ghz (or more if you want, very easy and safe to do so), so it will keep up with those 2 cards in crossfire, and i know you will be as hapy as can be.