Agree with Wildman. A friend of mine just added a 7970 to his i5 750 and it made a huge difference. Whichever way you look at it, you will need a new card, so why not buy it and see how much of a difference it makes? You could pick up a new GPU, RAM & AIO CPU Cooler now, then update the rest later if you feel you need it.
And unless you REALLY love to clock the pants off your CPU, any RAM faster than 1866Mhz really is overkill/willy waving.