Which CPU -- E8600 or Q9550?

[BH_Vfw_Dudley]

Gentlemen, your opinions please! Your advice would be appreciated.

My Win7 Prof system is a somewhat old ASUS P5Q 775 motherboard with an E8600 Over-Clocked to 4GHz cooled by a Cooler Master Hyper N520. RAM is 4GB of DDR2 800 MHz.
I do get fairly smooth video (30-40fps) on my GTX 550Ti with only some of the details turned down.

The E8600 is the top of the line for Core2 Duo with a stock speed of 3.33GHz.
However, I've acquired a Core2 Quad Q9550 which has a stock speed of 2.83GHz and expect I can OC it to at least 3.5GHz and hopefully extend the useful life of the system.

With RoF as the main application (yes... I have a sad life), which CPU would be better?

I'm guessing the Quad would be slightly better for the long run.
Ultimately though, I suspect my main bottleneck is the video card.

[BH von Güber]

I'd go for the quad core, personally. The ROF forums are (or used to be, at least) full of reports of trouble with dual core processors. I should think that since we're usually running the game, plus at least TS and TIR, maybe another overlay, etc. that something that does better in multitasking would help your performance. None of those things is a single big hit, but it is running multiple applications at once. Besides, a quad at even 2.83 should be more than adequate. My aging I7-920 runs stock at 2.67 and does very well.