RAM Upgrade on Alienware x51-R1

I have the first edition Alienware x51 that came out with the gt 545 and gtx 550 or something close to those graphics cards.

My specs are:

CPU: I3 - 2120  3.3Ghz  (2 cores) 

Memory(RAM): 6gb     [Don't know speed]

GPU: Zotac gtx 670 2gb Vram

Hard Drive: Internal: 1tb 7200rpm      External: 1tb Usb 3.0

PSU(Power Supply): 330 watts

I know my CPU and GPU are highly not matched properly but I am upgrading the CPU soon.

I want to upgrade my RAM, I don't know the highest I can get with the R1 so I would like some help I would to know the highest amount 

of Gigabytes I can get with it and Fastest speed also. Thanks