Not knowing what your reasons are for a laptop instead of a desktop computer, I must say, I would rather NOT edit video on a laptop, especially if it is HD video. Other than fairly simple cuts/transitions and titles, video editing requires fast and durable hard disk, plenty of RAM and a large display; certainly well beyond 17". While you definitely CAN edit video on today's laptops, it is something that you would want to do only if no other option is possible.

If you are buying a computer whose primary purpose will be video editing, the money you saved for the laptop will certainly buy you significantly better performance and larger display than a comparable laptop. You'll get better video card, more RAM, proper hard drive (a fast 3.5", rather than a slow 2.5"), and likely at least 20" of screen real estate.

As for your specific question, my experience tells me the following: if you're buying an Windows laptop, more RAM definitively improves your performance much more than faster processor; Windows likes to take up as much RAM as is available by the computer. On the Mac, the performance improvement with extra RAM isn't so dramatic, although I believe even there, you still get slightly better performance with complex software (such as Final Cut, Premiere, etc) with more RAM and slower chip than less ram/faster chip.

You definitely want to use a desktop over a laptop. I use a monster laptop for some small video projects (i7 1.66 ghz, 6GB RAM, GT360 1GB video card, SSD hard drive) and it's much slower than my desktop with similar specs. Everything on a laptop is scaled. Power Supply, bus speeds are generally slower and HDD RPMs.

I also use Vegas. A PC with a nice video card (research this because you don't necessarily need a $400 gaming video card), a solid CPU and a ton of RAM will generally cost less than a comparable laptop.

I agree with Vasic 100%.