Save your money and put it towards your next build like hackman said, or let your wife go shopping! 775 died when 1156 was released. The CPU might be worth upgrading if you had a cheap dual core CPU, but that's not the case.
I would purchase the 4Gb of RAM, try to overclock the CPU a little, and call it a day. Ensure the RAM is compatible with your current setup. Hopefully you have a 64-bit OS that can take advantage of it. You already own a nice Zalman cooler so see what you can get out of your Q6600 and dreaded 680i. There's no reason to waste money on a Q6700 - QX6850, or downgrade to a dual core CPU.