If you can wait 2 months, get the Q9450.
If you have a Q6600 already, a Q9450 isn't worth it IMO.
If you want a Q6600 and can wait til April 20, it will price drop to $220.
Clock for clock, the Q9450 is 15% more powerful but at 220 vs 360 USD, it is 63% more expensive.
The Q6600 is tried and true and a G0 should be able to hit 3.4-3.6Ghz easy on a good board.
Q9450 OC-ability is unknown, but assumed to be as good, so you might see 3.6Ghz stable OC's, though this would mean a FSB of 450Mhz, which might be harder to attain.
Q6600 has 2x4MB of L2 vs. Q9450's 2x6MB L2
Q6600 has default 266x9 multi vs Q9450's 333x8
Q9450 has SSE4.1, Q6600 doesn't, but most software/games won't take advantage of this for years.
Q9450 is 45nm vs Q6600 is 65nm. Q9450 is 95W and Q6600 is 105W. Q9450 has less heat production and power consumption.