Okay so im building a new system and it would really help to save money by buying a cheap board (ASUS M4N68T-M) or would buying a more expensive one (ASUS M4A87TD EVO) be worth the extra money?
Also could if i get the more expensive one, can i buy 2gb RAM now and get another 2 later and be the same speed as a normal 2x2gb dual channel memory?
One last question, could I buy a cheap brand of memory like Avexir or Mach Extreme?
That depends on the rest of the components you'll be using with the motherboard.
You probably would want to list your whole setup to give us a better view of your point.
IMO, without looking into the details, any motherboard with single GPU and 2-4 RAM slots configuration will perform relatively similar.
As for the memory issue, it's better to get a manufacturer's dual channel set than gambling over compatibility issue. Manufacturers already tested their dual channel RAMs pre-shipped, thus ensures the availability of the dual channel feature. You can get a dual channel using 2 sticks of RAM from the same brand with the exact spec. But even with that, there's no guarantee that it will run at dual channel mode.
Last answer for you last question: Yes, you can. But for the safe side, better not.
Being a minimalistic rig in terms of the number of components used, I'd say the ASUS M4N68T-M is more than enough. Using only a single piece of RAM with 1 slot left is indeed pretty minimalistic. Why not go for a dual channel?
Considering your screen resolution, your GPU and CPU is more than enough for the CoD and Crysis.