what PSU do you have and what resolution do play at? 2x 8800GTX should be playing games at a very good performance level. I assume since you posted on the Nvidia forum you want Nvidia options only or Nvidia & ATI? SLI/CF only or will a single card work?
Can you over clock the Q6600? It's on an Intel 775 CPU socket that is no longer supported. I would not spend money on any CPU upgrade if you can OC, since all the 775 CPUs are overpriced do to lack of supply and high demand before they are gone. Your best long term bet would be to upgrade to an AM3, 1156, or 1356 CPU & motherboard.