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.
Check out the conclusion section of this article.http://www.tomshardware.com/reviews/game-performance-bottleneck,2738-16.html