Precision M6600 memory configeration

I want to upgrade my dell M6600 ram, what options do I have in terms of maximum memory, speed, memory types, etc..

I currently have:

 One Intel Core I7-2720QM (2.20GHz, 6MB cache, Quad Core)

memory : 4GB (2x2GB) 1333MHz DDR3 Dual Channel

I aim aiming to upgrade to at least 16GB ram, shall I keep the existing (2x2GB) and buy 2x8 GB modules at 1600mhz (making a total of 20GB) or shall I go for 2x8GB at 1333mhz DDR3?

there appears to be alot of options and i'm not sure which is the best.

thanks