I agree with getting more RAM and a new GPU. Although if you have to limit spending, I would go for a bigger GPU first and then more RAM down the line. Almost nothing uses more than 4 GB of RAM, especially gaming.
Another option to consider is getting a new CPU cooler, CM 212 Evo comes to mind, and overclock your CPU as well as adding on a bigger GPU. Something in the 7870 range should be plenty to run Far Cry 3 and have it look great.
Also, if you are looking for overall performance boost, SSD all the way. Best/most significant upgrade available to any PC right now.