Do you have a certain preference as far as AMD or Nvidia? Also, what power supply are you using on your build, so we have an idea of what to shoot for?
Most people will recommend running one more powerful card over multiple weaker ones. There are some quality issues with Crossfire/SLI that just don't exist on single cards.
I personally prefer to add cooling fans to a case instead of relying on just the one that comes with it. Can't go wrong with more airflow.