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The upper graphics card can even be up to 280mm in length. If you remove the HDD drive bay (and go all SSD & NAS) you can install a card up-to 425mm. Motherboards wise you can install quite a bit, as long as the form factor is ATX, Micro-ATX or mini-ITX. CPU cooler wise it can be a cooler with a height of up to 165mm, please do keep note of that. You secure the graphics cards or other cards with a thumbscrew. At the top side we can see one 120mm Pure Wing fan, it is included and installed for you already. In the topside upper mesh you'll notice mounting holes for other fans and radiator and/or fan solutions.
 

 
To the left you can see that you'll get 7 PCI slots to play around with so in theory you can house up-to three graphics cards in here, we'd recommend not more than two due to the compact chassis design though. Also your maximum motherboard size is ATX, typically these allow for 2-way SLI/CFX maximum anyway.

Mounting an HDD is simple, you mount the HDD in one of the three supplied trays, slide them in at the backside and secure these with the included (small) thumbscrews. There are three hard-disk trays split across modular cages, so 2.5" SSDs could be mounted in there as well.
 

The rubber inserts on the sides prevent resonating noises from the HDD. You slide the unit into the drive bay and secure them by lowering the thumbscrew to the left downwards (photo below). 
 

 
As shown on the photo below, you may house 3 HDDs (3.5"), and at the rear side of the motherboard tray you can house two SSDs (2.5").
  

 
Here you can observe the backside, here you slide in the trays and secure these with some thumbscrews. Overall it works fine and your HDDs hardly resonate with this mounting methodology.