Your not only looking at 2 more cores, but you also get K10 and 45nm combined with a much higher clock speed. Even if your program can't use the 4 cores, it will still be much faster on the Phenom II X4.
I doubt cores will make that much of a difference, the much higher clock rate, newer technology, denser chip and other things will be the ones that will make the difference. A lot of software can't even use 2 cores, newer software can utilize two cores, even newer software can utilize two cores more efficiently, the newest software is starting to utilize 4 cores but very minis-cure, so the cores wont affect you nearly as much as the other increases.
The op said what program he uses. He said what he uses his machine for. It says on the website that the app supports multicore cpus. IF it supports 4 threads then it will a HUGE upgrade. And if the app does not. Use a app that does. Its that simple. For video encoding go quad core. Its that simple.
Since i specialize in low cost computing solutions for my clients:
For video encoding work more cores is best : in low cost solutions you'd best go with Athlon 2 X4 620 - the P2X4 925 even better if within your budget hehe Even a locked multi Deneb is waaaay more clockable than any K8/Agena i've ever worked with!
When you guys are done with your domestic dispute I'd like to submit my input, lol.
If you much video encoding, especially HD, go for the quad core ASAP and start saving huge amounts of time. It's a significant improvement. I'm starting to do more video encoding and my e8400 can't keep up fast enough. I'm ordering a Q9550 this week.
x.264 video encode benefits from more cores. Based on the same Handbrake settings between my E6600 HTPC and Q9450 primary PC, the scaling is nearly linear; meaning the extra two core in the Q9450 resulted in almost double the frames processed per second.
I'm really interested in video encoding benchmarks of the 6-core i9 Gulftown.