850w can handle a 5970 no prob. With a 1500 budget, it would be worth it, as long as none of your other components are super expensive.
A 5870 would be a great choice for now though if you want to save money. You might not even need the 5970s extra power.
I would get the 5970 ( that way you don't have to worry about upgrading your graphic card until DX12 is released and if you have a dual pcie 16 slot motherboard you can get a GTX 260 for phys-x). You have the money so why not live a little! You deserve a HD 5970.