Sell the HD4850 and get an HD5770 or HD5850. The HD5770 wont be a huge upgrade but it is DX11 and 1gb. It also should be pretty cheap after selling your other card. The HD5850 will be a very nice upgrade but is also quite expensive. With the x4 2nd pci-e slot you may want to avoid crossfire in the future so that would lean me towards the HD5850 for an upgrade that will last.
Yes you would need a better PSU to crossfire but with your second slot being x4 the advantage of a second card would be quite limited.
If your current PSU is running an HD4850 fine then your PSU doesn't really need to be replaced.