The seagate is ten dollars cheaper, and has the same specs as the hitachi. I recommend it just for the maxblast 5 software. I use it all the time. Takes a few minutes to download, but well worth it. Hope you can get one of these brands of ps: corsair, antec, seasonic, enermax, ocz, or xfx. Can't stress the power supply enough. I've had cheap brand ps smoke out on me in a few hours. Like smelling a dead skunk.
Yes. Read your motherboard manual, especially the memory section for instructions on installing two ram sticks. I always do a bare bones test on wood or cardboard before mounting the board inside the case. Just the cpu/heatsink, ram, video card, and keyboard to enter the boot order, etc. I touch the case connector pin cluster with a flat screwdriver and move it around until the board fires up.
No. It's a better board, but you still need a good power supply. Stick with your first board selection and get this power supply: http://www.amazon.com/Corsair-500-Watt-Certified-Compat.... It will power the 6850 and the rest of your setup. Run your setup for awhile without overclocking. Overclocking is never guaranteed anyway. I don't use it anymore on my system. I prefer stability, and I don't want to get a premium ps. Mine is only 400 watts, but with your 6850, the 500 watts is better.
The a-power is junk compared to the corsair. Either the gigabyte or asus board will work fine if you want sata 6, which I use with my ocz ssd. If you get the a-power and it dies within a few months, don't say I didn't warn you.