At stock speeds maybe was a bit bottleneck, but not much.

A dual core E8500, or E8600, can reach the same performance of a 775 socket quad if you can overclock it over 4.400 Ghz, however you need water cooling to archive that without burn your CPU.

But ClOD dont use a quad core CPU at full, so you can reach the same performance of a 775 socket quad in ClOD just reaching 4 Ghz on air cooling.

I did a little bench and here is the result, I tried reach the max eyecandy at playable frame rates, actually is in the speed of my VRAM where the bottleneck are in my system, specs in my sig.

Settings :

http://i54.tinypic.com/1vmvd.jpg

Maximun torture London, at that eyecandy at low altitude I have a minimum of 18 FPS, is not bad to fly arround and watch but dont try have combats there, and no CPU bottleneck :

http://i52.tinypic.com/25fq9ms.jpg

Free Fly England, flying over the country at low altitude I get a minimum of 34 FPS, not bad at all, at that eyecandy is amazing fly over there, very pretty landscapes, very playable at all, and no CPU bottleneck :

http://i53.tinypic.com/334sxfm.jpg

Bomber Intercept Channel, over water there no problems at all, with 9 bombers and 6 comrades, at that eyecandy, I have a minimum of 56 FPS, and no CPU bottleneck :

http://i52.tinypic.com/2hrfriv.jpg

So I guess that dropping 300 bucks for a Quad 775 maybe is not a good idea, you can save that money for a better upgrade.

Hope was informative :)

Edit :

That bench with Quad and ATI 5870 would be amazing Gunslinger :D