The new video card would be a huge upgrade
What games do you play or want to play and at what resolution?
I would try the video card first and see the results, check out Tom's Hardware Graphics Card Hierarchy Charthttp://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
Just a huge upgrade
What Power Supply do you have?
Then I would do the CPU.
I really don't understand the need for a new motherboard.
The increase from 6GB to 8GB would be small, but for the cost, reasonable. It is recommended to match the exact part number of the memory you are now using.