What kinds of things do you need to take into consideration? PSU? CPU? Mobo? All of the above? I am using an XFX 7850 DD, i5 3750k, 650w Corsair PSU. I'm not going to be doing this at the moment, but I was just wondering about this as I'm newer to building a PC.
You don't really want to add a second GPU if you can help it. You don't gain 2x the performance (very little gain in some games), you have to deal with driver issues, and you often get microstuttering. It is easier to just sell your old card and buy a new one for most people.
In no specific order:
> GAMES, i.e. which games you specifically play and how they scale with dual-GPU configurations, some sort of benchmarking/proof that they scale well.
> Motherboard/CPU - should be able to support at least PCIe 2.0 8x/8x on two lanes.
> PSU - should be a high-quality PSU capable of handling the second card.
So yeah, you need a very good reason to go dual-GPU. Otherwise stick to the golden rule of buying the best single card that you can afford, go from one single card to another.
As long as you have a 2nd PCI-e slot with enough room you should be good to go but as others have said, sometimes it's worth it to just get a better single card. Although lots of people crossfire/sli and get good results. It depends on the game and the support. Just do a quick google and you'll find answers. My eyes would never be able to deal with ANY micro stutter so I avoided crossfiring my 6870 and just went for the gtx 670.