I would go with 1 gtx280...and get a sli motherboard so you can pick a second one up later. the 4870x2 will suffer if the game is not coded for crossfire. and 1 gtx280 will beat 1 4870 (if crossfore isn't supported then the 4870x2 just becomes a 4870 1gb).
but I'm not a fan of 2 gpu's on one card...
you knock the 4870x2 for crossfire and then go on to give bad advice, on buying an sli motherboard, get a grip