generally speaking, you'll get more OC headroom with a lower multiplier but that extra headroom may not allow the 'slower' processor to eclipse it's big brother. most good mobos can get a q9000 series running with a 400mhz fsb with hardly any trouble, so the q9450 should easily hit 3.2ghz while the q9550 can be expected to hit 3.36ghz. the q9550 will probably need a voltage boost and may warrant an upgraded cooler to cruise at 3.36ghz. so, since 3.2ghz is your target, i'd recommend the q9450. be sure to check the forums at anandtech.com, xbitlabs.com, and hardocp.com to get more hands-on feedback.

don't wait for a price cut on the q9450 -- it's supposed to be eliminated from the quad lineup and the q9550 will drop in price to about $280 retail by the end of July.
http://www.dailytech.com/Intel+to+Cut+Prices+on+Existing+CPUs+and+Introduce+New+Parts+in+Q3/article11843.htm