I have the A8-3800 and 8 GB running at 1600mhz. The speed is the key factor here not the size unless you run very heavy programs. If your motherboard supports higher than 1866mhz go for high ram. Because the system memory is used as video memory. And the faster the ram the better frame rate and if you overclock you will be able to push even better video performance.
Actually I am crossfiring with a 6670. SInce I will be mostly gaming on this should I go for 8Gb. Does my resolution matter as well? I am playing on a 32' 720p HDTV.
My browser tends to use 3-4GB by itself, as well. Firefox with 200+ plugins, usually running a 30+ tabs. I'm a mutlitasker. If you can, get 8GB; if not now, you will put it to use later, in a new build, or if you convert this build to a server - it can't hurt!