minimum requirements for playing crysis 3 as recommended by crytek is a dual core cpu along with 3gb ram and atleast a nVidia GTS450/ Radeon HD 5770.
I have ran crysis 1-2-3 on my older system with a core2duo e7300, 3 gigs of ram and a gts 450. even on the minimum settings and a desktop resolution of 640x480, the game tend to give massive jerks at some places.
For u to start with, i would suggest to go with the ram, upgrade it to atleast 3gigs since i think u r using an x86 system and adding any further ram will be useless since ur system will only utilize 3 ( os limitation).
if that does not solve, u will have to upgrade ur cpu as well.