It depends more on the strength of you graphics setup than whether its crossfire/sli. With only a GTS 250 I doubt you'll see much of a difference unless you're on one of the very few CPU dependent games.
If you want to see the improvement bench you CPU before and after with 3dmark. Or compare your new quad to a Athlon II x4. Its the same chip as the athlon x4 if its anything like the Phenom II x3s.