Yep NightShadow has the right idea. It's all about the time frame.
If you plan on upgrading ever year or so then just get a cheap dual until the quad you want is released (not that the Q9450 is going to be much better than the Q6600 imo.)
If you want your PC to last for the next 3 or 4 years then you don't want a cheap or expensive dual for that matter, you want a quad as long as you can afford it.