Literally Double FPS Well probably 90% more FPS
wrong!the 5000 series and previous cards did not scale very well,depending on application/game,maybe 60-70%,since the 6000 series,scaling has improved almost to 100% depending on game/application,but most anywhere between 90-100%
if you are running eyfinity is definetly better,even a 60-70% increase is nice for eyfinity.My 5870 crossfire setup(previous cards)running at 5860x1200 ran very well.definetly better than just 1 card.
you will just need to experiment with the settings to achieve what is best for you.
Higher FPS at same settings or same FPS and more Eye candy,GL