I also advise getting a single card when possible. Some games can't use CF or SLI like GTA4. Other games have horrible scaling like WoW. (it's getting better with patches, but it still has a long way to go.) A single card won't have these issues.
If you research your games and know they handle CF well and are willing to risk any new ones that pop up will to, get the CF setup. The 6870s CF really well, and out perform two 5870s. There is no shame in getting a GTX570/6950 however.