Yes, you can put a x1 card into a x4 or x16 slot - it will work fine.
Note that the x1 slot and one of the x16 slots will be covered by the triple thickness video card (assuming you put the video card in the primary x16 slot, which is usually a good idea). However, you can use the x16 slots.
Why do you need a sound card? The sound chip on that motherboard is 7.1. Unless you need something special, I'd suggest you use the on-board sound.
You could find a different motherboard with on-board Wifi - that would eliminate the need for the Wifi card.
I don't understand why you would want a soundcard - Unless you do require it - if you don't need it, the onboard sound is good enough, you will not hear any difference if your using headphones/speakers
my brother has this one and it is incredibly good for its price, and the money that you save with buying this motherboard you can sink that into a better gpu, maybe a gtx580? (personally prefer ATI, but were not building a rig for me )