Those are similar specs to my low end system, except I had 6 gigs of ram and a 4870. Have you tried downgrading your drivers? I verified that I was running the 10.12 drivers on my 4870. It seems that most of the people having issues are using 5000 series cards.
Here's the link for the 10.12 drivers if you want to give them a try.
http://forums.guru3d.com/showthread.php?t=334401
I'am on the 11.2 drivers now.
I can use every driver,11.5,11.5a,11.5 RC3,11.3,nothing fixes the FPs problems.
Weird thing is,there are people who can play max settings with a HD5770 while others have huge problems with this graphics card.Pretty weird.