Just be aware on the differences on the i5 2500K vs the i5 3570K; the 2500K is older, discontinued and runs hotter. However, it does bench quicker at stock and overclocks "better" than the 3570K. As bn3 hints at, Sandybridge CPUs will work in Ivybridge mobos and vice versa. But be aware that you won't be able to use some of the Ivybridge "extras" when doing so.
For what it's worth, both the Z68 (Sandybridge) and Z77 (Ivybridge) chipsets have a technology called Smart Response. This allows you to use a low capacity SSD (<32GB) as a cache drive for a mechanical HDD, and effectively gives you a hybrid hard drive.
There is no advantage to going for more than 4GB on a purely gaming rig. Most games are still 32bit and often hard coded to use no more than 2GB RAM. However, if you expect to be playing with photoshop or reencoding video too then an extra few GB will help. A 4GB kit (i.e. paired 2GB sticks) in a motherboard that takes 4 DIMMS will allow future expandability without the need to replace.
If you know the games you'll be most likely playing compare the benchmarks of the AMD HD 7870 and nvidia 660 Ti for those games. But at that price there is no "wrong" choice.
