The E8400 has dual cores @ 3.0 GHZ 6mb L2 cache, 
the i5 750 has four cores @ 2.66 GHZ 8mb L2 cache

If only it were simple as looking at the specs....

however once in awhile you run across something that helps. From the following website

www.hardwaresecrets.com/article/801/4

comes a little nugget - Core i5-750 achieved an overall PCMark score 4.85% higher than Core i7-920 (which runs at the same clock rate), 34.25% higher than Core 2 Duo E8400

Other than having two similar systems to do actual render tests I can't tell you what percentage increase in render speed you would get, but I'm betting it would be significant enough for you to take notice.

Graphic Observations: Many problems can be solved by reading the manual Not all answers are equal Some problems don't need answers