Question about cases help pls :)

Solution
The NZXT can handle up to 325mm cards WITH the HDD cage in. A vanilla 7990 is 12'' or about 305mm, meaning they will fit even with the cage in.
As for slots, you should be fine.
I would not use one 7990 let alone two. For years "the 411" on CF was "it's catching up to nVidia but don't use more than 2 cards. Then the poop hit the proverbial case fan when these articles broke proving what peeps were reporting subjectively for years

http://www.anandtech.com/show/6857/amd-stuttering-issues-driver-roadmap-fraps/6
http://www.pcper.com/reviews/Graphics-Cards/Frame-Rating-Part-3-First-Results-New-GPU-Performance-Tools?page=2#comments
http://www.tomshardware.com/reviews/graphics-card-benchmarking-frame-rate,3466.html

Now AMD has done very well in improving things since then but not to the point where I could comfortably recommend doing CF ..... and 4 AMD GPUs ???? If ya go that way, I'd stick with just 2. here's the skinny on what the latest beta drivers have done / not done.

http://www.anandtech.com/show/7195/amd-frame-pacing-explorer-cat138
http://hardocp.com/article/2013/08/01/amd_catalyst_138_beta_frame_pacing_crossfire_driver/#.Ufsxbm0piX8

Case wise, Id look at the new Phanteks Enthoo