So I recently installed my second HD 4870...just to play with it :P
Well I know my PSU might not be strong enough, as it whistles with both 4870's attached.
750w PC Power & Cooling PSU-
Which may be the answer to this question, but I figured I'd bring it up anyways:
I bought Crysis recently just for some eye candy. Playing with one 4870 attached I can play with all High Settings (low,med,high,very high) with no graphical lag, tearing, etc
Although, when I use Xfire with the second card attached I do experience graphical lag in High settings..figuring I'd be able to run Very High Smoothly and not doing so was a disappointment.
But this is my first trial with XFire, so I am not too discouraged.