If I can make another suggestion, consider holding out for a good i7. It'll cost you a bit more than what you plan but you'll be able to go longer without upgrading again. You'll be spending about $180 for a Q9500 or $275 for an i7 960, but the 960 will give you a 50% performance boost and save you HOURS of waiting over the course of its life.
I might not be the best person to listen to when it comes to this sort of thing, though. I'm typing this on a brand new MacBook Pro and will go home tonight to mix on my i7 desktop but I'm gonna pay for two slices of pizza in quarters, so... yeah.
The first part of that is true and you'd also need to upgrade your RAM, reinstall Windows and all your software, and all that crap. The second part... you'd be OK. It is a pain in the ass but worth it if you're impatient and hate waiting for stuff to happen.