I know that if you want to use the 7th PCI-e slot on this motherboard, you need to have a 3rd generation Intel Core processor, and Celeron won't work. That is an extra $100, so I thought I'll buy a second MB with that if I was happy with my experience.
About even using 6 Cards, I got some powered riser cables to power them separately as I read people have fried their MBs by not doing that.
Are you sure you need a non celeron processor?
The bottom PCIex16 slot on the Z77A-G65 Mobo has a sticker on it which reads: "Due to bandwidth allocation limitation, it is strongly recommended for users to install 3rd Gen Intel Core processors to enable functionality in the third PCI Express slot"
I think I misinterpreted their "recommendation" but the manual claims the 3rd generation Celeron enables it as well. The power will be split across 3 slots. If you are using 1, it acts as a 16pin PCIe, if you are using 2, it acts as 2 8pin, and if 3, the top one slot will be 8pin and bottom 2 will be 4 pin.
I setup my rig. I have 5 out of 6 R9 280x cards working and their optimal hashrate. What I notice is that the card connected to the 2nd PCIe x16 is not detected. I am using all powered risers and am using latest Catalyst Beta drivers and SDK with Ubuntu Saucy PXE booted over network. I think somehow, the 1st PCIe x16 is acting as a 16 pin so it is not distributing the power to the other slot enabling it. Currently I am trying to troubleshoot this problem. I was not going to use 7 cards BTW.
I have a celeron G1620 which is a Ivy Bridge CPU (http://ark.intel.com/products/codename/29902/Ivy-Bridge) so it should have enabled all 3 slots. I wonder if there is a BIOS magic.