Heres a view of the motherboard, looking toward the IO ports.
The socket for the CPU is at the top-left of the picture, the 5 PCI slots are at the top-right. To protect the underside, the board is rested on the packaging it came in.
Check that the IO ports all fit the holes in the rear of the case, punch-out case plates as required. We wont fit the board just yet, but we can prepare the case -- its easier to attach the CPU, heatsink and RAM whilst the board is outside of the case.