If I were building a system build like yours, I would recommend you get a single GPU, just because it'll work better than a CrossFire or SLI setup of similar cost. I've heard numerous complaints before that people have gotten two cheaper GPU's, put them in CrossFire or SLI and regretted it afterward. Often, you'll see a noticeable drop in framerate if you use two cards that are each 1/2 as powerful as 1 card.
However, you may be asking yourself, "if having two GPU's creates lag as opposed to one more powerful GPU, then why are there high-end cards that are dual GPU's?" The fact is that, with CrossFire and SLI, you have only a relatively small bus going from one GPU to the other, which tends to bottleneck the communication between the two. Beyond a certain level of GPU power, though there is so much parallel traffic going across the GPU that having two dies is actually an advantage, in spite of the bottleneck on the CrossFire/SLI bridge. This is why you see high-end dual GPU's at the upper end of the market.
Also, if you are looking for CrossFire or SLI, I would say that a GeForce GTX 970 or a Radeon R9 290 would be the minimums in their respective classes; however, before you do CrossFire/SLI I would recommend you consider a dual GPU solution such as the GeForce GTX TITAN Z (which is all right) or the Radeon R9 295X2 (which is better than the TITAN Z and currently holds the prize for the fastest desktop GPU in the world).