unless you need cpu cracking the cpu isn't important at all. 2gb of ram are also sufficient. All that matters is vram. You don't need a fast harddisk either. Even PCIe x1 would be enough per card. If you have the money you should buy 290X instead of 5870: https://hashcat.net/forum/thread-2798-po...l#pid17284
With four cards one 1200W ps might be enough: https://hashcat.net/forum/thread-2798-po...l#pid17486
With four cards one 1200W ps might be enough: https://hashcat.net/forum/thread-2798-po...l#pid17486