Dual gpu cards tend to be more trouble than they are worth but also lower clocked compared to single gpu cards. I have owned a few in my time and sure they are great for those who can keep them alive while enjoying the benefits of crossfire or sli without needing to deal with multiple cards but they have a short life span.
The GTX 590 is clocked at 630 mhz and even though they have two 580 chips on them ,two GTX 580's will beat the 590 every time and that is because the single GTX 580 is clocked anywhere from 772 mhz to 850 mhz depending on the card you buy. If I had one slot available I would have a hard time deciding between a GTX 590 and a single GTX 580.
Multiple card configurations will always out perform a single card in value. The point is if you start your new build PC with multiple GPUs it is hard to upgrade your system because when you start using more than two ePCI slots your performance decreases and your only options is to sell the 2 used cards for as much as you can, which wont be much b/c you are buying older GPUs to begin with. If you buy the most powerful card you can afford and then a few years down the line when you need to upgrade you can buy the exact same card(which will be much cheaper then) and your system will be on top again.
1 powerful card = Cheaper upgrade to future proof system