impressive figures for the xfire 5770s... considering the total cost of both cards, i can see why going down this route is so tempting! hmmm...
5770's beat the 5870 overall. A couple of titles (borderlands comes to mind) suffer from driver issues (low min framerate) out of line with general performance that will more than likely be ironed out soon enough. Have a look at the guru3d december vga charts, i seem to remember the crossfire 5770's beating the 5870 in 7 of the 9 games tested and even beating the 5970 in 2.
Here it is
http://www.guru3d.com/article/vga-charts-december-2009/