How to physically configure graphic card for higher yields? If I were to purchase a motherboard with 7 pci-ex ports. Could I then attach 7 graphic cards and run 7 cgminer processes? One for each card?

Or would a better build be to group cards together with SLI/Crossfire?

If one of these cards generally gives 1200mhash, would i be correct to assume that 7 would give me 8400? Or is there some overhead issue here I'm not aware of?
A single instance of cgminer handles multiple GPUs, so there is no need to run separate processes.  Enabling SLI(You aren't using NVIDIA cards, right...) or Crossfire will decrease mining performance, so leave it off.

There is no overhead associated with running multiple GPUs on a single system, unless the cards are physically spaced so close together that they overheat and throttle. 7 1.2 Mh/s cards would be expected to produce 8.4 Mh/s total.

At this point, investing in new GPUs for mining is questionable, and you should only consider GPU mining for scrypt-based altcoins such as Litecoin, as ASICs have made GPUs unviable for bitcoin mining.