One question: would you choose to have 1 really good video card or on top of my crappy 550ti I buy another decent one and do dual?
I always find one better;
SLI and crossfire is dependent on software.
Some games do well, some don't. Optimized drivers always lag game roll outs...
New cards are more efficient...by the time you think of buying a second card...you can buy a new one that is twice as fast and cost less, and uses less electricity to boot.
Only go SLI if you want insane resolutions and high settings NOW.
Not some time in the future, example; pushing 3 or 5 monitors for 3d driving or flight sim.