I recommend single cards, i'll tell you why.
If you are a gamer, you will most likely focus on how good your FPS is. Crossfires can result in micro-stuttering, and drop your fps below 20.. You don't want this to happen. Single cards use less power, they have less noise and less heat. 3 big things. Also, if you think your video ram will double because you have 2 GPU's, well your wrong. Video Ram doesn't double with multi-gpu's it uses the Video Ram of only one card, and leaves out the other one. Let me tell you, there are very good single cards out there that can run all games at max settings. Like the 780ti, or radeon r9 290x. If you're thinking of getting a crossfire or SLI, well, im going to say don't.