-Because it's pretty much the best PSU you can buy for consumer systems. There are others that run neck and neck with it, but not for that price. You can save about $50 by getting the X750 instead of the X850, but a fully-loaded Z68 SLI system (once you get that 2nd GPU) can require about 700W, and I wasn't sure whether you would be happy with only a 50W buffer. More power isn't always better, from a heat/efficiency standpoint. According to
ATX specs, a "typical" running load should fall between 50-75% of the PSU's rated wattage.
-The Gigabyte board uses the Z68 chipset, the MSI one uses the P67. That means the MSI one
won't be able to do
SSD caching nor
Quick Sync. You might even be able to save a few more bucks by only getting a 32GB SSD instead of the 64GB one, but I figured you would want the maximum cache size possible. The biggest potential drawback is that the Gigabyte board looks like it uses BIOS instead of EFI, meaning your max C: drive size will be limited to 2TB.
-I recommend DDR3-1333/1600 RAM because testing shows there really isn't that much difference in performance between 1600 and 2133 speed RAM (but there
is a difference in price). The brand of RAM matters more for the guarantee the vendor offers than anything else. So long as you choose ram that is rated at 1.5v or less, and has CAS latency of 9 or smaller, you'll be doing pretty well. There are performance vendors with fancy heat spreaders, (the G.Skill ripjaws, for instance), but you will need to make sure that the board and case have room for odd-shaped modules before you purchase them. Good brands (in alphabetical order):Corsair, Geil, G.Skill, Kingston, Mushkin, Samsung. Within each brand, some lines are still better than others, so don't jump at a $20 'value' deal just because it has one of these names.
--Patrick
EDIT: @SpecialKO - There are multiple PSU calculators out there. Just search for "PSU Calculator"