Fella, I have both 24" i.e. 1920X1080 and 22" 1680X1050.
Tried Crossfire and single both.
Crossfire on dual core 10FPS higher than Single card on Quad.
However for descent gaming all you need is 30FPS-35FPS.
Which was also achieved on Dual Core PC with 4870.
But when used on Quad I could do Antialiasing, Very High/Ultra Settings, etc which was completely possible on Dual Core (i.e. E8200/8400), hence Quad Core (i.e. Q9450) was a real achiever here.
Try Benchmarking new games on Quad, they will perform much better don't try on old lame games which are not optimized for Quad Core and will only utilize 2Cores.
Adios!