So this Christmas season, my friend will be spending $300 to upgrade his Acer Aspire M3470-UC30P. Basically, it has a 350w PSU, 2.5GHz Llano A8, and a 7750. I suggested him to add another 7750 and change the CPU to an i3. He mostly plays SC2 and pretty much nothing else.
Oh, in that case, I'd go with an i5 3570k ($170 at microcenter), the Hyper 212 EVO ($26 at newegg) and then whichever Z77 board suits you in terms of features and looks. I'd recommend an Asrock at this price point.
That should leave you with just under 100 left over, if he feels the need, he could get an i7.