As for selling your parts, how much is a three year old cpu/motherboard worth, in my eye's about $100. 6gb of ram another $50, video card $100. You might be able to get 300-350 for the set if you find someone gullible.
For $600-700 you will be getting maybe 10-15% higher frames then if you bought just a $200 video card. If you buy a $350 video card then you will have higher or equal frames as if you upgraded your whole system.
For gaming, you almost can’t beat the Intel® Core™ I5 2500K matched up with a good P67 chipset based board. The price on the 2500K is very attractive and the performance that you can get from it without even overclocking simply put can’t be beaten. Once you start to overclock it you will be very happy that you picked up this processor.
So the better idea right now is to pick up a card like the GTX 570, and a good cpu cooler so I can overclock my Q6600 is a better idea than spending the money to do a whole upgrade. At least in the short term?