The manual identified by Daniel Beck suggests four dual-width graphics cards can be mounted as shown
The HD 7970 is dual width but uses a sngle slot
Putting a HD 7970 in the bottom slot looks a bit tight to me, with those other connectors under part of the HD7970.
The case also needs to be able to accommodate a dual-width card in that position. It looks rather unconventional to me (but I don't build gaming PCs).