For the sake of gaming you can ignore the i7 series and AMD 6 core and here is why;

1. AMD hex core CPU's are bitch slapped by Intels i5 series in nearly every test.
2. i7 CPU's have Hyper Threading which is only used in things like video editing/encoding etc. Most games do not support this feature.

My advice would be as follows;

CPU = Intel i5.
RAM = 4GB.
GPU = nVidia GTX560Ti.
MOBO = Gigabyte.

Make sure to buy a good quality PSU too something by Antec or Corsair are good names to go with as an example. As for what to put it all in by something that suits what you need it to and fits within your budget.

JGM.