Up to you whether this merits the money. Personally I would get the 2.8GHz and for 600.00 extra buy a W3680 and plop it in for the 6-core 3.33GHz goodness. If you are not comfy doing that then get the 3.2 as you will be stuck with it.
I am running a 2.8Ghz system right now and my feelings are you will see a bigger difference with a fast hard drive and enough memory so disk swapping does not happen.