Do you think we will see a time when two SLI graphics cards will be required to play games or will there always be a **one card solution** to counter-balanced high end games for people with low/mid grade gaming rigs?
If the trend for 2 vid cards gets forced upon the market, cause like PCIe has been(though I see why it was) then yes we will need SLI or Crossfire. However I can't see anyone doing that in the near future as long as SLI or Crossfire does not follow some standard of equivalency like PCIe or other form factors follow a standard approach so that they are more stable.
Notice that right now they both do it differntly, to do the same thing. If they both worked on the same level so that you could Crossfire on a SLI board or vice versa then the advent of every new gaming rig being dual card would be more likely.
Now we also have to take into account that there could be new tech that comes out that changes the way the GPU works, say GPUX2(two GPU on one card) that is mainstream like AMD and Intel Procs dual cores are becomming. Just like we are reaching the heights of our processor abilities, we will eventually need to switch to a new GPU form factor too.
The possibilities are pretty big as far as I can see.