1. No, 6gb/s will not help that. The current drives don't even max out 3gb/s.
2. Ram isn't really that important because performance gains are minimal.
You should buy what you can afford. 1600Mhz is good. If you plan on doing
extreme overclocking, then the 2000Mhz+ will be better, but most 1600mhz ram
can be overclocked to about 1800Mhz, even 2000Mhz.
Also on that GA-P55A board, if you do use SATA 6gb/s, then you won't be able to do SLI or CFX because it shares the same PCI-E lanes as the GPU. Which also means your GPU will run at 8X instead of 16X. Also, it's not true 6gb/s. It's more like 4~5 gb/s because of the integrated PCI-E controller on the CPU. If you want true 6gb/s for a P55 board, then you have to get a board with the NF200 chip or got the X58 route.