If it's primarily for gaming, i5 without a doubt. 8320 may be better in highly-threaded non-gaming stuff, course it draws more power though so costs a bit more in the long run.
EDIT: I failed to specify which i5 there... reason being, it doesn't really matter. Grab whatever there's a deal on and take the 3570K if they're the same (or near same) price. 3450/2400 would be just as good if you're not overclocking.