Your build already won't work well for your stated use. You say you want to do Crossfire, but you have picked a motherboard that runs its second PCI-E slot at 4x and a POS PSU that will most likely blow up just powering a single 5850.
That POS PSU should be the first thing to go, even though it's essentially free with the combo. A quality PSU is the key to a stable system. That PSU is anything but a quality unit. I would use this GPU/OS combo instead: