How can you have 5GB of ram? 3x1GB+1x2GB? 2x2GB+1GB?
E8400 is a dual channel ram processor and you should definitely have pairs of the same type of ram installed. I suspect you have a typo there though and you mean 8GB (4x2GB) which was normal to have in a computer built around the time when E8400 was new.
I have the same system as you except a Radeon HD7950. It does get laggy from time to time for me as well, but disabling the ingame anti aliasing and using Sweet FX antialiasing helped a lot for increasing my frames per second. Also, it felt a lot smoother when I put the game on my SSD rather than where it was first, on a 7200rpm HDD.
As Kalston pointed out in some other thread, you can make a HDD much faster at reading data if you short-stroke it. Some info about this right here.
http://forums.ageofconan.com/showthr...=183372&page=8