It's best to use 80% of the rated PSU wattage. 4 cards suggests a usage of 1040 watts at maximum (260 *4)
You'll want to go low power for the CPU, possibly underclock it, and then run a flash drive to save $..
With that power supply you should use only 3 and then CPU mine,
Downgrade the CPU to a sempron, it will consume less power to hande the high wattage GPU miners.
Have you thought about the spacing of the GPU cards? You should get a mobo that supports sempron with the capabilities for 4 GPU cards (PCI-E x1 and x16).
As far as CPU mining goes, You'll need a little more RAM to power 6 1GB threads along with the OS.(Maybe 8GB RAM)