A first generation i3 as compared to a second (or third, I'm not exactly sure which) generation pentium is tough call to me.
While the i3 has HT, yes, the pentium has the updated, smaller architecture that allows for better integrated graphics and quicker processing. I would actually see the pentium outperforming the i3 in this case.
I would try to find at least a third generation i3, in my honest opinion.
Like this; Newegg.com - lenovo G580 (59359688) Notebook Intel Core i3 3120M(2.50GHz) 15.6" 4GB Memory DDR3 1333 320GB HDD 5400rpm DVD±R/RW Intel HD Graphics 4000