Q9550 or Q9650. But 505090 is correct. Especially if you are overclocking past, say, 3.2 GHz., The improvements just aren't worth it.
I have a Q6600 and a Q9550, both OC'd to 3.6 GHz. Except for video work where the Q9550 has a small edge, I cannot tell the difference between the two without benchmarking.
Save your money and get an i7 or wait for i5.