Well to be honest sicne you plan to game the E8600. Especially if you plan to OC since it has a 11x miltui which will benefit you in the future. I have seen the E8600 on ari over 4GHz.
But depending on how long you want the system to last a Q9550 would be good too. Any idea how long you want the system to last?
Subjectavly, Quads just run smoother and faster. IF the system is meant to last around 2+ years, go with the Quad. If its a holdover for a newer system when i7 becomes affordable, get a duo.
If you are going to CrossFire the P5Q Pro will work at PCIe Gen2 x8x8 with your HD4870s. If you want x16x16 you might consider the AMD 790fx (w/sb750) mobos and a Phenom. I think the 'single button' AMD Fusion system OC thing might apply to them.
It's pretty easy to OC the e8400 - and you can bump up the HD4870s with a number of available tools ....
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