1333 RAM would give a decent benefit and will be able to be used when you upgrade the cpu/motherboard.
if you get a decent heatsink for the cpu and overclock it, its not so much getting more but help prevent the cpu bottleneck the gpu. but you will get more out of it, for an example (and these are numbers i am pulling out of the air) if you OC by 20% you may see a 3%-5% increase in framerate. ( again, i am pulling these number out of the air)
getting the 6870 will provide the best increase in performance. overclocking the cpu will keep it from getting limited and getting faster RAM will help load games and levels a bit faster.
btw, since you're fine with PCI 2.0, you ought not have a problem with PCI 3.0, have you considered a 7850? it will be better than a 6870: