Not entirely sure for Crossfire but SLI has issues with scaling for multi-GPU set ups. The simplest issue is that it is rare that two graphics cards double the performance. You would also require a suitable motherboard which supports a multiple graphics card set up.
I believe the general consensus is that a multi-GPU set up isn't really worthwhile. Not many games are well optimised to take advantage of multiple graphics cards, and for approximately the same price as two graphics cards it is likely a singular graphics card of the same price would be a lot easier to use.
Take this as being a gaming specific response.