i went from an e5200 @ 3.75 GHz to the i3-2120
and i got a very nice improvement.
on what gpu? what games?
dual for a dual + HT not a great upgrade imo. dual for a slower clocked quad is worth it. Save for another month or two, sell some dvds you won't watch on ebay, try for a cheaper mobo, B75 not Z77 (as you won't be OCing) and you'll make the difference up.