it depends on the person. if you don't like to deal with multi gpu configuration then single gpu is the right solution for you. as for me i prefer single card solution but i don't mind going for multi gpu solution in the future.... if i have the money for it
It would depend on which single card you intend to get. It is generally less hassle to get a single more powerful card than 2 lesser cards, means you don't have to worry if a game will support SLI or Crossfire properly, or SLI/Crossfire driver problems. I would take a single 6950 over say 2 6770s any day. I would get similar performance either way, but much fewer issues with the 6950. I personally would only bother going dual GPU only under circumstances where I would want huge amounts of GPU horsepower ie. triple monitor gaming, or trying to run Crysis 2 on ultra with DX11 and trying to get something close to 60FPS. On a single display that isn't ultra high resolution you really don't need that much graphical muscle for the vast majority of titles.
If your starting a new build it's almost always, if not always, better to get the best single card you can afford. You can then add another card later. Buying 2 cards initially for the same money limits you in the future. If you already have a decent/good card and your motherboard support sli/cf then it makes sense to go multi.
I've personally had 2 dual GPU setups (3dFX & current 4870s), my brother has one (6950s 2GB) as well, a few guys from work too (560TIs & 4890s) and aside from very minor glitches where all you needed to do was turn off SLI/CFX until driver was fixed, they've been awesome.
My only real recommendation is never settle for cheap/weak setups, because IF there are any problems, you're stuck with a weak single card until everything is fixed. The setups I mentioned are based on cards that are still pretty strong in a single configuration (ok, my 4870s are definitely getting old, but even a single one gets the job done if temporary).
Like supernova said, to anyone wanting SLI/CFX, much better idea to go single strong card for a little while and get the same one when budget permits than go right away with 2 weak cards.
It depends also on what you are planing to do. If you are going to do a little gaming like cards, puzzels, and media then one gpu Now if you want all the heavy gaming and run everything on ultra plus more than one monitor then two or more gpu's. What it all boils down to is what are your needs and how can I make them happen. Is it going to be one gpu or more than one? Having one has its pros and cons same as two gpu's. In my case I bought one gpu and had problems so I bought another not knowing what I know now, but the second gpu help me to game at med with good fps as to the one gpu where I couldn't play even on low with bad fps.
Crossfire/SLI is best reserved for upgrading IMO. I initially bought the HD6870 when it first released last year and was pretty satisfied with it, but after I added my second HD6870 it was a world of difference and now my setup can destroy basically any game out there now.
But much like a lot of other people are saying save the crossfire/SLI for after you build your system and find a powerful single card solution.
depends on your budget, pc specs, future upgrade plans.
i prefer single powerful gfx card setups. less hassle, less tweaking settings, less maintenance.
multi cards have their advantages, imo they're better for upgrading a current build. but for starting a new build a single card is better.