Intel Core i7-920 vs i7-960

Hi
I'm buying a new computer (intending on getting a Dell Studio XPS 9000, once finances become better), and I'm pretty sure of what I want in all the available customizations - except for one thing.

I can't figure out if the $400 some odd dollar upgrade from a Intel Core i7-920 (2.66GHz) to a Intel Core i7-960 (3.2GHz) is worth it. This stems ultimately from the fact that I don't really grasp what clock speed is; and I don't want to pay that much more if I may end up not utilizing the increased speed. What I plan to use the computer for will /work/ on both... but would I notice the increased speed with the better processor? Should I get it just because I go for years between buying new computers and things in the future might require faster speed even though my current uses don't?

If I'd actually use it, and it only helps minutely, I'd still get it.. but if it's only useful for people doing massive video rendering or computer astrophysics (which are about the only two things I definetly won't be doing), then I might as well save the money right?

Newbie Needs Help