You have 2 x16 slots and 3 x1 slots. You will need x1 risers for the x1 slots. It should hold 5.

Thanks for your reply and help!
So, those connectors labeled PCIEX1_1, PCIEX1_2 and PCIEX1_3 are meant to be connected to a GPU, right?

If I had 5 of these:

http://media.sapphiretech.com//images/prods/2044/21227-00_R9_290_4GBGDDR5_DP_HDMI_2DVI_PCIE_C02_635180480190970430_600_600.jpg

In order to work properly I'd just need 2 of these:

http://ecx.images-amazon.com/images/I/311TDzGFsmL.jpg

and 3 of these:

http://ecx.images-amazon.com/images/I/31D92TWV20L.jpg

Together with a power supply unit, cpu, ram, hdd. Do you think this can work? Or, perhaps, some software problems may arise in using 5 gpu's?

Sorry for sounding like an idiot, but I've never done this before and the most manuals on mining are all like: "oh yeah, just connect some cables, download some shit and press some buttons dude..."