The 6850 only requires one plug.
It really depends on which 4800 series card you have. For instance, if you have a 4870, neither card would be an upgrade in speed.
Also assuming you have a decent quality 500w PSU to run the newer cards. It should be at least 80 plus certified.
As for the extra 6 pin PCIe plug, you can always get an adapter like this. http://www.newegg.com/Product/Product.aspx?Item=N82E16812189120