Sabertooth is overrated and expensive. Don't spend a good chunk of your budget on the motherboard, on a gaming rig of any budget you always want to pour your remaining funds into getting the best GPU you can get as that's the most important part. CPU is important but everything else is pretty much an after thought.
and whats the difference between the motherboards?
You posted in the format in the OP that you wanted the option to add another card later on. You want a board that runs dual cards @ x8. That Gigabyte board in that build I posted runs dual cards @ x8. Click on the link to that Gigabyte board and read the specs on it.