I would always opt for the better single card instead of two cards (well, the x2 is technically one card, but it's really like two). Not everything scales well with two GPUs or more. Plus, if you have an SLI-enabled mobo, you could always upgrade to two 280s.
Performance-wise, the 4850x2 is a bit stronger in many cases, but I'd still go with the 280.
Well it's always better to have one card if games don't support Crossfire (or SLI). Since you would just be getting half the power of your expensive card.
itd be easier if it was a 260 or a 4870 x2, lol the 4850x2 is better than the 260 for sure, and the 4870x2 is better than the 280 in many cases. It changes from game to game. But between those two specifically, Id say the 280