Hello,
The first thing you ought to consider is the motherboard and the processor. When you have found the adequate one then do let me know the model number of the motherboard then I may be able to help you. Without the model number just guess work won't do much.
Anyways, DDR3-1333 or DDR3-1600 should be a reasonable speed and the size of the memory will depend on the applications you intend to run.
Let me know if you need any more info.
Thanks