There are some statements in this thread that are not always correct, but that is dependent on what NLE is being used.
An 8MB video card (with no 3D) is enough for video editing.
This is not true for Pinnacle Liquid Edition. It uses the GPU to render some effects and requires 64MB minimum. 128 is better, but 256MB addes additional functionality in Hollywood FXs for real-time preview. For others, this may well work. Windows Movie Maker, Vegas 5, and Premier Pro do not specify. I do not work with them or hang on their boards, so cannot tell you if it makes a difference. With ATI 9600XT cards reaching the $150 mark (9800 Pro is in the same range - I bought 9600XT from Newegg as a refurb in May at $114), you could afford something faster. Check the web boards of the NLE vendors as some cards work more consistently than others with that software. ATI works well with Pinnacle, but that could also be because of their hardware partnerships.
well paraphrasing... The faster chips only have 10 seconds difference in render time
One benchmark does not justify not buying a faster chip, but do remember that you need to consider the savings over the lifetime of the computer. If one chip is faster at rendering, allows more effects (such as PiP to be displayed in realtime) and has a faster workflow. It may pay for itself in 6 months or less.
Xeon/Opteron? Does the editing suite say it recommends a dual? If not, it would be a waste of time. Prescott over Northwood, maybe. Some suites do better with Prescott. Tom's actually got better rendering results in Studio 9 with a Prescott at the same clock speed. The 3.4 and 3.6 are being discounted (see main page or the Inquirer?). AMD? Check the recommended page of the NLE and any support boards. Some NLEs have specific support packages (iow, the vendor has those in the lab - a lot to say for resolving issues when you know the vendor has one too.)
I have a dual myself. Not way over what a 3.0 could do until I start adding multiple effects to the same clip(s) in a 1.5 hr project. Then it is only 5-20% faster than a single. But, that means less frustration waiting for the machine to catch up. But, my NLE supports duals and many do not.
But, before you spend more money on a CPU, check what you have for HDD. If you plan to do a lot of editing, 500GB is a good idea. My current project for a drum corps is at 175ish GB. And that is SD, not HD.