I would suggest the gpu to a much newer one. Are you using xp or windows 7? And is it 32 or 64? If it is 32 then 4 gigs is your max ram. If it is 64 you could go as high as 32 gigs of ram (over kill really). 8 gigs for windows 7 64 seems to be the sweet spot so a newer gp and bumping up to 8 gig ram should see some fps improvements.
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.