OP, what clocks speed, mobo, video card, and memory type do you have?
I ask because going from an OC'd Q6600 to a PhII X4 is a sideways move, the X6 is a little step up, but primarily only in encoding/heavy multitasking. Don't get me wrong, they're great chips and I have one myself, but if you have say a 3.2ghz Q6600 and 4gb of DDR2, then getting a PhII X4 965BE or so isn't going to be very much different at all.
Probably best at this point to try to sell your current CPU/mobo/ram and going for at least an i5-760, or preferably at least an i7-930 (microcenter special). If you can wait a short while, perhaps the new 2600K will be to your liking.
I'd just think that you would be disappointed to spend $400 and not get a real leap in performance. If your Q6600 is stock for some reason, then grab a nice 120mm heatpipe cooler and get it from the 2.4ghz to at least 3.2ghz (this is really easy with most Q6600s), it will be night and day, the C2D/C2Q series really comes to life starting at 3ghz or so.