I wouldn't take a Q9550 past about 3.0 GHz with the stock heatsink. The heatsinks that come with the 45 nm CPU's are half the size of those that came with the 65 nm Core2's. They just do not have the reserve capacity to work much above stock frequencies.
For overclocking information:
This should be your first stop.
HOWTO: Overclock C2Q (Quads) and C2D (Duals) - Guide v1.6.1
http://www.tomshardware.com/forum/240001-29-howto-overclock-quads-duals-guide
This should be your second stop. You
need to know something about thermal management or you can fry your CPU. It's actually kind of difficult to fry a modern CPU, but it is possible.
Core 2 Quad and Duo Temperature Guide
http://www.tomshardware.com/forum/221745-29-core-quad-temperature-guide
Third stop will be a guide for your particular motherboard. Google is your friend.
For anything higher than about 3.0 GHz, you
will need better cooling. One of the best is the ThermalRight Ultra 120 Exteme (abbreviated "TRUE"), but it is a little expensive. Here are two under $50 heatsinks that are pretty popular:
Sunbeam
http://www.newegg.com/Product/Product.aspx?Item=N82E16835207004
Xigmatec Dark Knight
http://www.newegg.com/Product/Product.aspx?Item=N82E16835233029
They both require a somewhat different approach to applying thermal compound.
Suggestions for applying thermal compound:
http://benchmarkreviews.com/index.php?option=com_content&task=view&id=170&Itemid=1&limit=1&limitstart=5
And they are pretty large, so they might not fit inside your case.
Go through the first two threads. Do not do anything until you have a good idea about what you are trying to do. Once you have definite questions (you will, you will
), come back and ask.
Keep in mind that these are guides, not cookbooks. YMMV. Your Mileage May Vary. Because of all the variables, you may not do as well as someone else with a similar system. Or you might do better.
----------
Overclocking since 1978 - Z80 (TRS-80) from 1.77 MHz to 2.01 MHz
GA-EP45-UD3P, Q9550 OC'd to 3.6 GHz/TRUE, 4 GB Crucial Ballistix,
GTX260-216, WD 2.64 TB HD's, Corsair 750TX, Antec 900 
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