Crossfire - no windowed mode allowed so if you plan to play some games and multitask you cannot do window mode in crossfire
Sli - finally supports windowed mode, however I heard that scaling isn't good as it requires nvidia to have a profile set up for SLi in order for it to worker properly and see gains as to a single card
Either way it's a good alternative to upgrade. Some people prefer to have 1 single card even if they lose marginal % of performance but won't have to face these cons
Personally, I was always tempted to Xfire my 5850s but instead I sold that on craigs and forked out 100 for a 6950 (flashed to 6970) whereas I had to fork out 130 - 150 for a used 5850
If it's cheap go for it
Posted via RS Mobile