With just under $100 you can get something like these (There are others to choose from):http://www.newegg.com/Product/Product.aspx?Item=N82E16813130556http://www.newegg.com/Product/Product.aspx?Item=N82E16820145315
You will upgrade ram to DDR3 1333MHz and your Q6600 will be back in business!
I have done an upgrade for a friend who also had a Q6600 and we did something similar.
The Q6600 is in the same categegory as the i3 so you will get plenty out of it yet!
I suggest not going for the DDR2 it will just maintain a mediocre state that you will not be very happy with.