A HD7850 will be roughly equivalent to a GTX570, but use a LOT less power. I'm not sure I'd rely on that old BFG 450W PSU to power it though. Here's a 520W modular Seasonic that is only $60 right now: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151093
although there's shipping costs too; make it $66, which is still a good deal.
If getting this PSU limits you to the HD6870, that's still a big jump over what you have.