ill try to answer what i can.
1. don't count on newegg reviews and comments to base your purchases off of because anybody can make a comment, positive or negative, without having purchased it or without knowing squat about that part.
the person who commented on that is somewhat right though, the lynnfield die only has 16 pcie lanes, so crossfire and sli can only run at x8/x8, whereas with an lga1366 cpu w/ and x58 mobo, two cards would run at the full x16/x16 bandwidth.
however, as you can see from this article.
ATI HD 5870 Scaling Performance: X58 vs P55 Showdown - AnandTech :: Your Source for Hardware Analysis and News
yes, there would be a minor bottleneck with xfire and sli in a lga1156 build, but not as much as you might think.
with that said though, if you can afford to get a x58 i7 build, i'd say go for it.
as far as mobo suggestion; if you can tell us how much you're willing to put towards a mobo we can make suggestions.
2. i think my above response kinda answers this.
3. well the 6 core 980x is for the 1366 socket, but the i7 930 is quite a fast cpu so i don't know if youll need to upgrade in 2 years, especially if you OC it.
4. i think you're less likely to have major issues with name brands, so i guess yes. but also yes, 12gigs is kind of alot of ram. you could go with 6 gigs for now and see how that works for your needs, you could always add more later.
this kit would be good.
Newegg.com - OCZ 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model OCZ3X1600R2LV6GK
7. a 1000w corsair is actually on the overkill side. you can get a 750w and still be on the safe side.
Newegg.com - CORSAIR CMPSU-750TX 750W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS Certified Active PFC Compatible with Core i7 Power Supply