Taking this build as the example (http://pcpartpicker.com/p/ADR3)
as it is the better of the two.
1) That processor isn't as good as the i5
2) You only have 1 stick of RAM so you're not taking advantage of dual channel memory.
3) The memory you've chosen isn't as fast
4) Yes the 7950 is better, but not by much. The 7870 XT is based on the 7950 architecture, if you overclock the 7870 XT it will perform the same as a stock 7950 for $80 less.