@TonyD and others:
That's exactly what I meant with diminishing returns. Your CPU (although the same operating clock speed) is better because its more efficient while your graphics card is a little less (no offence). It's to be expected that you will get less FPS increase when raising your CPU speed. I expect the difference will get bigger when you upgrade to a 6950 or 6970.
After seeing TUSA/TX-Gunslingers beautiful post in another tread (
http://forum.1cpublishing.eu/showthread.php?t=22417) I saw that his GPU (5870) peaks often at 100% for longer periods while mine peaks at about 71% for only very short periods. If I can lift this percentage to 100%, this would mean at least a 40% increase in performance which would be clearly noticeable in a heavy game like this. Besides, I can use the power for other things too (like heavy multilayer Photoshopping). So, I've enough excuses
. However I will probably first await the next patch.