I've recently upgraded my editing system. and I'd like to put down a few of my thoughts since the upgrade. My previous system was:
Motherboard: ASUS P5W DH Deluxe
CPU: Intel Core 2 DUO E6600 (2.4 GHz) @ 3.06GHz
RAM: 4GB OCZ Platinum DDR2 800MHz @ 667MHz
GPU: NVIDIA 7600GS

I upgraded my CPU to the Q9450 (2.66GHz). It is a drop in replacement to the E6600(2.4GHz) and the price is right: got mine for $279 at microcenter.com. Subjectively, my working experience is faster. CS3 is snappier and due to the extra cores does a better job of encoding footage while I edit without killing my editing. This is overclocking the Q9450 to 2.88GHz and the E6600 overclocked to 3.06Ghz. Do to my motherboards age it does not support a proper voltage for me to take the Q9450 to a higher clock.

Why not the Q6600?
Originally, I planned to upgrade to the Q6600 which is cheaper by about $100. But my brother beet me to it and so we had a face off: E6600 vs Q6600 in Premiere Pro CS3. For the test we each rendered out a section of SD footage and rendered a scene in a 3D graphics package. We also did some work in After Effects CS3 , but didn't use identical operations. We found that the Premiere and 3D times were nearly identical, the quad core actually lagged just a tad. However, for some heavy lifting in AE the Quad did do better, not nearly twice as fast however. This isn't surprising, based on what I read from Adobe. It seems that though some operations use multiple cores, basically Premiere doesn't use more than two at a time. The other two could be used by another application however. We tested the E6600 overclocked at 3.06 Ghz against the Q6600 running at stock (2.4GHz) and the 2 cores were faster, by a significant amount.

Ethan's in IL right now, but when we can I'll try to get together a set of actual benchmarks and times with the three chips.

Upgrade path
If you're like me and don't have money coming out your ears or would rather spend it on video cameras, 35mm adapters, and lights with wattage output that would cause Al Gore to loose his negative carbon rating. Then you want a system with a steady upgrade path so you can buy it a little at a time.

My upgrade path from here will be to a Motherboard that supports both nethalim and DDR3 and then move to 64-bit Vista and 8GB RAM sometime after CS4 comes out (Nov. 08?).

Conclusion
I recommend the chip as a good intermediary upgrade if: you have a motherboard that already supports it. The benefits of this chip are its 45nm architecture and 12MB L2 cache. The cons are 8x clock multiplier. Basically, 45nm means it runs faster at a lower clock speed. I didn't think it would be noticeable but it is. The L2 cache is good for applications like Photoshop and Premiere Pro. The 8x only matters if you're going to overclock it. (For reference the E and Q6600 have a 9x multiplier and are a little easier to OC).

I don't believe it's a good time to upgrade the motherboard at this time because: Nehalem, a new chip archatecture incompatable with current sockets, is coming soon (4Q '08, that's why the Q9450 is has dropped in price.) and DDR3 is still maturing (and expensive). A note on Nehalem, Intel is planning to release the Extreme series of chips first, which cost a premium for the speed.

If you're about to do a project and would like to give it a little boost. I am impressed with what the Q9450 did for the price.