Always go single-card if possible.
-Lower power consumption
-Easier to upgrade (it's hard to sell 2 old low-end cards, and if you absolutely want to you can add another card in the future, since you already haven't used CF/SLI)
-Not all games utilize CF/SLI 100%
-You can get a lower-W PSU -> saves you $
-Less stuttering with single-card
and so on
Go single card