The AMD build offers support for AMD's upcoming CPU, Bulldozer. It supports both SLI and CrossFire, as well as enough money for a great CPU cooler. The Intel build offers the best performance by a very noticeable margin (the i5 2500K is currently the best CPU on the market for gaming), but sacrifices the 8GB of RAM you had in kind as well as SLI support (only CrossFire support). It will support the upcoming Ivy Bridge CPU's but only the 4 core version, not the 6 or 8 core versions. It does include a great CPU cooler as well, but if you want SLI support as well, you may have to give the CPU cooler up, but that would still make the Intel Build the best. You choose.
The X4 OCed (you can easily reach 4.0GHz with good cooling) will last you a good amount of time until the need to upgrade. I would choose between ASUS or Gigabyte. MSI boards are fine, but ASUS and Gigabyte seem to build higher quality boards. When it comes to quality and performance, ASUS and Gigabyte boards do not differ by much. However, I found ASUS' UEFI BIOS the most attractive BIOS. It is easy to navigate as well.
The newest chipset is the 900 series. I would go with that considering you get the added feature of SLI without bad Nvidia chipsets. In CrossFire, the PCI-E lans would be 16x and 4x, not 8x and 8x. The ASUS would be at the same level as the Gigabyte, so if you want to save $5 get the Gigabyte. if you want a the same features as well as a UEFI BIOS, get the ASUS.
I am not sure what the difference is, but from what I've been told, there is no noticeable performance difference. As for RAM, 4GB is more than enough for most tasks. If you ever need more in the future, you can add more. I would just get 4GB of RAM to start with.