That motherboard support up to 1333 MHz memory and use dual channel memory.
So get memories in pairs with speed 1333MHz. How many GB you should get depends on for what you want to use that PC for. I guess 4GB may be enough. In that case go with 2x2 GB 1333 MHz memory kit. This allow you to upgrade to 8GB by buying 2nd 2x2GB kit (same type) later if you need to.
If you need more memory go either directly 4x2GB or 2x 4GB.
To matter of brand get any of reputable Corsair, Crucial, Kingston, G.Skill, ... Which ever is cheapest. You won't see any difference.
EDIT:
Forgot maximum memory supported is 16GB. So ultimate maximum memory you could get is probably 4x4GB, not sure if that motherboard will support 8GB sticks so rather use 4x4.
So if you tell us what you want to use that PC for we can help you decide how many GB you should get.
EDIT2
:
What OS you use. Because if you use 32-bit OS it have limit of 4GB ram usable. Bit lower in fact but that is about something else.