yup the pci express slot can be 1x or 16x, the mining speed will be the same. but you need 16x to 1x riser cables to attach the 16x gpu to the 1x pci express slot.
if you wanna start out with a gaming rig, just get the asrock 970 extreme 4 as recommended. it has 2 pci express 16x slots so you can mine with 2 cards without the need for cables. just get cables and add more gpu once you are satisfied with mining profitability. otherwise just connect the crossfire cable and you have a monster gaming rig!
the asrock has an external on/off switch on the board as well as a reset switch so it's easier to use this without a case if you are saving on cash.
crossfire does not help in mining. just install the cable if you are playing games :)
personally I am using Asrock 990FX Extreme4 and 2 units of Gigabyte 970A-DSP for mining. the Gigabyte is cheaper but it does not have an on-off switch on the board so I have to use a screwdriver head to turn it on. I do not use a case for my 2 rigs.