You know how there's always a notch in the card connection?  Line it up with the notch of the PCIe slot.  If you try it, it won't break anything.  It would be a problem, or more of a problem, to do the reverse.  Plug x16 device into x8 slot for example.  That would have its limitations. I haven't heard of wifi card problems with the 790 yet if that helps.
I have a optiplex 790 SFF and I want to install a PCIe-based WiFi adapter but I don't found the PCIe x1 slot. [IMAGE]
Where is the PCIe x1 slot for dell optiplex 790 SFF