Short answer: it depends on the software you run.
About the only reason to consider the Q6600 is if you run software that will take advantage of four cores versus two. Distributed computing (F@H), some video/music editing/encoding software and a few games will actually gain some benefit from four cores but even most multi-threaded software out there will only utilize two cores.
Since your cpu runs at 3.15GHz there really isn't a reason to upgrade for speed reasons.
I am nearly in your same position, I have an e6400 @ 2.8GHz (bumps 60C under load on stock cooling or I would go higher) and it does everything I need it to right now. I plan to upgrade my GPU within the next two months (I see you have already done this, nice 8800GT
) but don't intend to switch CPU until probably late next year (either Penryn quad or full refresh to Nehalem depending on when I want to go quad and pricing).
Of course, with $130 M0 e4500s floating around that stupid upgrade bug has been biting viciously these days...