And the decision whether to Xfire HD 4870's or one HD 5850 will depend on your power supply. 2x4870 in Xfire would be something like hd5870 but it still depends on weather your power supply can handle those 4870's or not
Be sure to check your sys specs and then post back
I understand that 2x 4870s is around the performance of a 5870. What I'm wondering the most is if DirectX 11/Power Savings/Future Proofing worth the extra money I will pay for a 5850 for less performance on the Dx10/9 side?
Then a 2x4870 will suit that system very good
Your CPU will handle it ( either the X4 or X6 )
Your Psu will handle it
As for the DX11 thing if you would consider a 5870 vs 2x4870 yes i would say go for 5870
But, there are few games with DX11 out there and developers are still making DX9-DX10-DX10.1 titles ( except for Just Cause 2 - it won't support DX10)