Overclocking q6600 on the system

and also using CPU zalman
http://www.ebuyer.com/140478-zalman-cnps9500a-led-aero-...

Zalman CNPS9500A-LED Aero Flower Socket 1156/775/AM2+/AM3/754/939/940 Cooler

What's the model of your motherboard? CPU-Z Validation, would help.

p45r2000 asrock

Well, in theory you have no FSB wall on that motherboard, so you can freely overclock it to whatever you want. Even 4.5ghz is not a problem to that motherboard. And for a good cheap quad-core 775 I would recommend a Q6600 (IMPORTANT, for best overclock-ability try to get the "G0" stepping as that one has a higher max temperature!) but if you have at least 120$ that you can spend on a processor, this CPU is for you(click here), I bought one myself, didn't get it yet, still didn't get here. It's an Extreme Edition processor, that means it has an unlocked multiplier which is good when overclocking, it is a very good processor, as it will easily overclock to 3.8ghz and actually can be as good as a i5 2400-2500 series. (Synthetic benchmarks). I'm getting one of those. Hope it helped you! (You will need a nice cooler if you want to overclock it, stock cooler get's you at around 3.0-3.2ghz no problem.)

Cheap and good:Q6600

Lil bit more expensive and amazing:QX6700

Other recommendations[If you're not in-to overclocking and want nice power and cheap processors(Doesn't mean that they won't overclock)](these are awesome prices for these kind of processors):
Intel® Core™2 Quad Processor Q9550 (12M Cache, 2.83 GHz, 1333 MHz FSB)
Intel® Core™2 Quad Processor Q9400 (6M Cache, 2.66 GHz, 1333 MHz FSB)