They don't actually use the ram per say, but they need it for allocating.
I also wouldn't put my money on the G55 being able to do 7 cards. It will do 6. It's possible they changed it with the Z87 but with Z77 the GD55 did 6 cards and the GD65 & MPOWER boards did 7 cards
Definitely get the SSD, small ones can be had some 50-60 bucks and they are a major relief when you are constantly doing things within your miners. Hard drives are such a pain in the ass. When you look at the cost of the whole rig, the extra speed for that kind of relief is just a small percentage.
Get a PSU that has 5 or 7 year warranty. You also don't need add2psu. You can make your own little adapters for $1
Don't bother with the large coolermaster fans or any PC fans. Just get a regular box fan.
Yes use powered risers for every card.
I assume you have a 20amp breaker to run this on? A typical 15amp isn't large enough for one of those rigs. Unless you are running 240v you'll need to hookup that rig to an outlet from either a garage, kitchen, or bathroom. Don't change the breaker size in your breaker box. You can run a cord from a large outlet that is fine. Use a 12 gauge cord for 25ft. Anything longer make it a 10 gauge cord. You'll also need special 20amp surge protectors.
I have ran a 6 card rig with 2gb and it worked just fine. So...