I just checked the power supply you were thinking about. It has only one 6 pin, 2x3, PCIe power connection. If you are going to install two 4850 video cards, then you will need a psu with two 6 pin, 2x3, PCIe power connections - one for each video card.
You might want to consider a top quality, brand name psu that is reliable, stable, efficient and certified. The upgrade is worth it.