Hmm with 2.6ghz you will be able to play a handful of games, Smash Bros. should work just fine for example, Muramasa the Demon Blade too, mostly games that are more of a 2D perspective though Rune Factory Frontier also worked okay when I had a 2.5ghz CPU. (That said I heard that i7s are very efficient compared to the Phenom I I had back then so you might get better results than me)

But sadly games like Super Mario Galaxy don't even run smooth with a i7 on 4.3ghz as of yet however Dolphin is in constant development so that might be optimized in the future.
Oh and having only two cores is fine, Dolphin can't effectively use more than two anyways and if you need any help with the configuration or anything just let me know here^^