I have an almost identical system (9550, p5q pro, 4870 x2, xigmatek cooler, vista 64, mushkin 4gb, corsair 750tx) and was able to easily get to 3.4. Its the most stable system i have ever built. The one problem i have with the 4870 x2 is that many games require updated catalyst drivers to run both gpus properlly. When they are working right, it easily runs almost all games at highest settings and resolutions.
As for the motherboard, i would deffinetly save some money and go with the p5q pro over a much more expensive alternative. Unless you are trying for a real high overclock, it should be great for you.