If you already have a lower end model HD 7xxx family graphics card then by all means crossfire, it's a cheap way to get competitive performance, all the 7000 series graphic cards are still relatively new so if you, say, crossfire a pair of 7750s you'll still get decent performance, and has the potential to match a GTX 650 Ti or a HD 7850, somewhere around that area.
However, if you currently have no graphics card at all, it's better to just get a single card, and in this case I think you should go for one of the R9's, they are a bit more expensive than older models but they do come with performance enhancements, most notable in 270x out of all the other revisions. That and in the future crossfiring these cards will be easier since they...