I get no lag, at full screen 1920x1080 with vsync, except for like 2 seconds when loading the launch pad. I've got 16gb of RAM and an i5 2500k OC'd to 4.5ghz also an Nvidia gtx 570. So besides bragging on the internet, perhaps these specs will give you an idea of what you need. Before you buy more RAM though, look at your processor as many others have said. And if you have an unlocked proccessor, definitely try to overclock it if you think you can do so without melting your computer.