Which Radeon HD 7870 card version will you be using in CrossFire?
There is the Radeon HD 7870 Boost Edition that is based on the AMD Tahiti LE graphics processor chip.
There is the Radeon HD 7870 GHz Edition that is based on the AMD Pitcairn XT graphics processor chip.
The Boost Edition card draws a lot more power than the GHz Edition card.
Do you know the brand and model of the graphics cards that you will be purchasing?
Do you have a specific budget limit that you want to remain within?