well the prices on the Q9550 just dropped on newegg.
it's 339$ compared to 329$ for the Q9450
the Q9550 is stock at 2.83ghz, and has an increased multiplier of .5x making it 8.5x
the Q9450 is 2.66ghz at stock, and has a multiplier of 8x
so basically, you get a faster processor at stock speed, and it has higher capabilities to OC. for a 10$ difference but i'm sure the price of the Q9450 will fall soon.