Based on what WonderSausage has shown us, it looks like a factory installed 4870 utilizes dual 6-pin PCIe power cables that connect to something on the motherboard (presumably dual 6 pin connectors). On my old SLI PC gaming rig, I used a pair of PCIe 6-pin "Y" cables to serve two cards (both with dual power connectors) from one pair of PSU PCIe power cables. Here's the product I used: http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=186&products_id=2664 I'm guessing, you could easily use this to add a second PCIe powered 4870. Those concerned about over loading the MP's power supply may have a valid argument but only if both those graphics cards were rendering 3D to the max while the CPU's were also simultaneously maxed. A very unlikely event even if gaming under Windows with Crossfire (since few games can saturate all that CPU). I can't imagine any circumstance under OSX where both CPU's and Graphics cards could be at max power draw. Certainly, single CPU systems would have enough power supply overhead to get away with this. The benefits of this would be the potential for Crossfire gaming in Windows and also huge OpenCL capabilities under OSX. Just a thought.