I know that the x58's support three channels of ddr3. But i was able to come across 4x2gigs of ddr3 and did not know if i should just try and sell the last 2 gigs or put it in the system. I tried looking around the internet to see what the answer would be but i figured id rather hear from something more direct. Does it make the memory run slower i really dont know.
Some boards work with 4 sticks of 2 gb, and some don't. I would use whatever memory the board maker recommends, and download the motherboard manual and check the possible memory configurations. Seriously, the x58 boards are so new, you'll get more reliable info by simply reading the manual.