Hmm...Dear I Have 2 Computers..
1. I5-750 (2.66Ghz), 4GB Ram Kingston, 500 HDD
2. I7-960 (3.2 Ghz), 6GB Ram OCZ Gold Series, 1TB HDD
And I Put HD5870 On Both System And Checked Performance...There Is Only 10 Fram Rates Difference Between Both Pcs...
So, I Suggest U To Go With..
I5-750 (2.66 GHz)
6GB Ram (G.skill Brand) Tri Channel
MSI Or Asus Mother Board (Both Is Good) Asus P7P55 LX
HDD Wht U Need....
OCZ 700W PSU
Sapphire ATI Radeon HD6870
And The Other Parts U Can Get At Less Price...
This Is My Sugg. To U....
Best Of Luck...