The MSI-z77a-G45 supports 5 cards out of the box and 6 cards max if you do a presence pin short on pci slot number 5. It will not run 7 cards. You need the GD65 to run 7 cards. This info is personally confirmed by me and many others on here. Do a search in the forum to find out how to do the pin short so you can at least run 6 cards.
On that note the G45 is a steal at $80, or even $125. You'll pay 160ish for the GD65 only to get 1 more usable slot. Some people really need that 7th slot to maximize physical space efficiency, while others just build more rigs.
You'll want to get 2 or 3 24cm riser cables to run the outter cards if you're going to run 6 or 7 cards so you can spread them all apart a fair distance. Too close together and they get hotter. The standard 19cm cables will do fine for the rest of the cards.
You only need 1x risers, and YES you WILL want them powered. Either do it yourself or buy them. If you are running more than 4 "big" cards on a mobo, the cables must be powered or you will eventually fry your mobo from too much load.
What about gd 55?
