What you have to do, is realise, whilst the faster it is, the faster it will be, technically a 2ghz dual core is 4ghz, and a 2ghz quad core is 8ghz, its just you have more than one.
On a Dual core, you could have 2 apps running with 2Ghz each, on a quad, you could have 4 apps running with 2ghz each.
It depends whether you are going to be multi tasking more, or playing games.
(BTW, 2ghz was an example)