Dual = (2·3.00)/(4·2.33) = .64x quad
Again, that's theoretically, with 4-core-aware tasks. For single- or dual-threaded tasks, it's just dual = 3.00/2.33 = 1.29x quad.
That of course is a simplification. Ever heard of multitasking? Even if you have several single-threaded apps running concurrently you benefit from multiple cores.