I have two GIGABYTE 7870 OC cards in Crossfire, and with the rest of my system it can cap at 765W. That being said, I have an 850W PSU.

If you're adding a second card, it'd be a safe bet to get at least an 800W PSU.

My specs for your reference:

Intel Core i5 2500K @ 4.3GHz (103.02 x 42) | Corsair Obsidian 650D Midtower Case | 2xGIGABYTE 7870 OC 2GB 1.1GHz Crossfire | ASUS Xonar DG 5.1 | Patriot Memory Sector 5 8GB 9-9-9-24 | Corsair HX850 Modular PSU | ASUS P8Z68-V LX | ADATA SX900 128GB SSD

As well as what the others have said the other thing is when it comes to crossfire or sli a lot of people assume that 2 cards equals twice the speed/power this is untrue, in reality you are getting MAYBE a 50% increase to your graphical processing power while also doubling the biggest heat producing item in your rig.

Power supply is a bit dodgy and maybe, if you're Crossfiring cards, you need a good good PSU with a lot of output. The most a 7870 will draw is around 300W(that's in benchmarks that push the GPU to it's absolute limit), with around 250W at full load in normal applications.

The PSU could handle it but I'd rather try with something like a 750W PSU or better.

Yes, you will see a performance increase. 7870 CrossFire Benchmarks In something like Skyrim, not so much. Scaling for the second/third card in Skyrim is absolutely terrible, less than 30%.

I think you would only really benefit if you used multiple monitors. I use 3 hd monitors, and have two GPUs. If you want an fps boost, I suggest overclocking. I have my GTX 660 super clocked, and it could easily reach 400fps on maxed settings.

It will improve fps but not so significantly that makes it worth it.
Unless you've got two monitors, then it's good to have two cards.
But really the amount of fps you'll get is not worth 200ish dollars.
And I dont think it will.