The HD5870 is about on par with the HD4870x2. It will also use much, much less power, give you DX11 compatibility and no crossfire scaling/compatibility issues. If you are will to overclock you could also save some money and go for the HD5850 which can OC past the performance of a stock HD5870.
I think I am going to go with your suggestion on the HD5870, but I need one last piece of advice. I checked Tigerdirect and Newegg, but I have no idea what I'm looking for exactly to ensure that my system will be running as smooth as it once did. I do see the list of choices but I don't know which one would be best for me. Thanks again!!