If you buying today than get what you want!! I'd consider getting the Xigmatek s1283 CPU HSF w/retention bracket for your OC'ing q9550. Or even the Arctic Cooling Freezer 7 pro for mild OC's if you like. The stock cooler should handle most situations, so you can decide if you want to get an aftermarket CPU HSF.
* Now I'm going to assume that your going to be running on a high resolution monitor with that 4870x2, right?? If your going to be running on a 1680 x 1050 resolution monitor than you'd be better off getting a single 4870. Save that $200 and use it later to upgrade the GPU in 6-12 months or so. If your running on a higher resolution monitor with resolutions starting at 1920 x 1200 or bigger than the 4870x2 will suit you well.
* I'm also going to assume that your using Vista/XP 64 bit OS?? If your not, than your wasting the extra 5 gb's of RAM.
I would recommend the Asus Maximus II Formula best mobo ever
At $270 shipped it should be!! For more than $100 less, any good p45 based mobo will do 95% of what the Maximus can do. Only if your into extreme OC'ing would I buy that board. I'd rather spend the $100 on a better LCD/GPU/Memory or whatever I think I need to upgrade. Just my opinion, but that is what I'd do.
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.
what monitor are you using? i'm trying to decide a 24in monitor (any recommendations for TN and non-TN? i tried to differentiate TN/nonTN monitors in newegg, but there was no option? can i tell the difference by the 160/160 vs 178/178 viewing angles?)
Most 800mHz DDR2 RAM will OC fairly well. The Cas4 stuff (4-4-4-12) is faster than the Cas5 stuff (5-5-5-18). To minimize headaches, I'd recommend sticking with stock voltage RAM sticks (1.8v) and get the fastest RAM at that speed, like the ones I linked above (5-4-4-12). Then if you need to OC them, you should have more voltage room to help you with the OC'ing (if needed). Most Cas4 ram is already OC'd and most of the time the voltage will need to be upped to get the rated speeds (most are 2.0-2.2v, if you look at the details).
The Asus Maximus Formula II mobo is quite expensive. I don't recommend it for most people, because you really don't get that much more performance out of it than a $140 mobo. There are options that it has, but it's generally for more extreme OC's, which most people won't even use!