Johnny,

There's a few things to focus on. Motherboard, cpu, and ram together, and its price per PCIE slot, psu price per watt (750w's are usually lowest here aside from getting a 220v server psu but those can have more headaches to get running), price per riser card. and finally price per card.

I'm running an asrock fatality mobo with 7 slots (not fully populated), my price per slot is $32 + risers.

Best card for ROI, 270, or newegg special on one of the 7870's. Build the rig on a boltless shelving unit with 12" deep shelves and use aluminum angle to hold the cards.