The parts look compatible and that video card can only run 1 monitor.
You could save a lot of money by using combos and going with an i5 quad system. You really don't need anything higher than that for gaming.
The delayed/slow alt tab issue on starcraft2 happens with my 5850s too but not with my GTS 250. Its a well known issue for ATI cards.
The 470 is usually better in performance but the 6870 is not far behind. The 6870 is definitely better when it comes to performance per watt though. However, for you I would go with the 470 because of the games you named. Blizzard games tend to favor Nvidia.
Maybe something like this...
You could save $20 - 30 by going with a cheaper GTX 470 but I really like this one
They're both quads with nehalem cores, the main difference is the hyper threading. I doubt you would have issues with processing even with ventrilo and multiple tabs opened on an i5.
Yes, they are the same. They are compatible but I highly suggest not getting an intel motherboard because of the minimal features. Theres really no reason to like them over superior boards put out by their partners like Asus and Gigabyte.
The difference between that Gigabyte GTX 470 and other ones are that it is completely rebuilt from the ground up (PCB and everything). Its one of the best customized cards ever (imo) and runs way cooler and quieter than the standard 470s. The core speed difference is very minimal and you can easily OC if desired.
FYI, I run 60+ fps everywhere except Dalaran (30-35 during primetime) & raids (usually around 50 fps), while having firefox open with a ton of tabs and Ventrilo running. And I have an AMD Phenom II X4 965 and a 5870 (at 1920x1200).
In other words, your concerns about not having enough processing power are unfounded. Any modern quad-core will do (either AMD or Intel) for playing WoW/SC2/etc. + multitasking.