personally, i'd keep that CPU. a 0.3ghz between the 3450 and the 3570 followed with a $200+ price tag for a 1-5fps boost (assuming this is for gaming) at best just isn't worth it. you're better off spending more on a better GPU, a decent case to fit it, and good PSU.
if you persist on upgrading the CPU, may as well pay the extra cash for a 3570K and OC it, assuming it's a Z77 motherboard in there. it's highly like it isn't.