8 GB RAM is plenty of RAM for all but the most demanding of applications. 16, GB is overkill for most situations, and realistically, no one ever really needs 32 GB RAM.
What will you be doing with this system?
In the meantime, the best memory for your system is 1600 MHz @ 1.5 volts with a CAS of 9-24.
Here's a quality low cost kit (8 GB) that meets the specs:
Corsair Vengeance Blue Low Profile 8GB (2x4GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit (CML8GX3M2A1600C9B) [CML8GX3M2A1600C9B]http://www.overclockers.co.uk/showproduct.php?prodid=MY-336-CS&groupid=701&catid=8&subcat=1517