You can't crossfire 2x 6870s in a Dell OEM system!
The power supply for a start in that computer isn't even enough to power a 5450. The RAM from what I can see on these specifications is only 512MB, this is not good. I suggest if you want to upgrade your graphics card then you are going to have to get a new build!
Also, there is only one PCI-Ex16-2.0 slot on that motherboard. You need 2 for Crossfire!