I checked out your motherboard and you're right, Q6700 is the highest quad-core your motherboard supports. In your case, you can try searching for it on Ebay as that's probably the only place you'd be able to find it at a somewhat decent price but I would recommend against this as a used CPU from Ebay is just never a really good idea because quality can't be guaranteed.
I think the best thing for you to do would be to wait until you can upgrade to a Core i-Series because any other upgrade your motherboard can support would be a parallel and you wouldn't really feel you're getting value for the money. That's what I did with my old motherboard that only supported an upgrade to a Core 2 Duo E6700 and nothing higher (Asus P5LD2), I just got rid of it and purchased a new motherboard, CPU, and RAM combo with a Core i3 530 because if I had bought the Core 2 Duo E6700 to upgrade from my previous Pentium D 820, though performance would have most certainly been better, it wouldn't have been as good a value for the money. And in your case, you already have a good CPU performance wise so you may just want to hold off on upgrading and wait to save up for the full combo; the Core i-Series is really worth it.