Is is worth it to build to i7 if you already have the Q9450? This is such an open ended question; hence, so many answer from various angles.
I'll key in on the word "worth". If you just have cash to burn for a gaming rig to support a hobby, yes it's worth it. If you're taking out school loans in college, then it's not worth it. For the gaming experience, the GPU is more of a determining factor tham the slight bandwidth difference between an overclocked Q9450 and i7.