GTX260 and 4870 are both good and comparably performing cards. They each require a psu with two 6 pin pci-e connectors. Your psu as none.
It is possible to use adapters which might come with the vga card. But--- if the psu does not have the pcie connectors, it likely does not have the power to use the adapters either.
Look for a quality psu. Good units come from corsair, PC P&C, antec, and seasonic. There are others. A budget psu is asking for trouble.
You will have to spend $80-$100 for a good unit like this corsair 650tx:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005