An athlon II 620 propus will do the job well. its affordable and performs admirably. if he wants to spend mroe cash, an Phenom II 955 or Core i5 750 would also do the job. but honestly, at that res and with that GPU they will be overkill.
what sorts of games does he plan on playing? an athlon tri-core would most likely be enough.
in that case, i'd say to go for an athlon 2 tri-core. only if he plans on playing CPU heavy games like GTA4/BC2/FF14 would i say a quad is needed. since its obviously casual gaming, a qaud would largely be wasted bucks, though it would certainly be useful if he wishes to spend the extra cash.