I agree on the P45. I have a GIGABYTE GA-EP45-UD3R MOBO and there is a great overclocking advantage on the board. I have a Core 2 Duo E8400 on it and have currently got it pushed up to 3.66GHz on stock fansink! Later on whe I get water cooling I am expected to try and get 4.00GHz out of it. My MOBO has 2 or 3 overclocking awards. And the chipset is a major contributor towards overclocking. The review I read on the board before I bought it to get rid of an old ASUS had used both the E8500 and the Q9550 to test the overclocking ability of it. It came back with great results. But the only problem they came across was a stubborn single channel memory configuration. Apparently the board wouldn't boot without a stick in the red channel. I have all slots used so I have no issue.
I also decided for P45 on my mobo, because its more recent process heats less and allows better overclocking.
But for a quad-core (you mention Q9550) I wouldn't take a core 2 and its associated Northbridge, as the Fsb is a bottleneck for throughput and latency. I would go to a Core i7 or an Amd, with direct Ram-Cpu connection.