I think, the upgrade offer with the free upgrade will end as soon as R7 is released, i.e. on April 30.
Regarding the hardware: I personally think RAM and Ghz would be more than just enough. i3 should be fine, too. Though with i5 or i7 you'd be more on the safe side. But I think here are some peeps around who can give better hardware advice on PCs. Anyone?