CPU upgrade help/question

My current setup:

Msi p6n platinum sli mobo, 8th kingston ram, 2x evga 8800gt ssc in sli, corsair 750w psu, intel e6420 CPU.

My question is: i was hoping to have enough money to upgrade my core components but unfortunately that didnt happen. I have just enough to upgrade my CPU to either a quadcore q8400 or a dual core e8400. My question is will it be a worthwhile upgrade over my e6420. I can't use anything but lga775 because I can't afford to upgrade the mobo. My CPU has always been my bottleneck as well. I was wondering if the performance boost will be worth the 180.00 I'm going to end up spending .

Thank you in advance!