I highly doubt I'll ever get a second GPU unless I need to, even then I doubt i'd pair my current R9 270 with it, I'd rather sell it.
I'll have £300/m to spend on whatever I like so getting the motherboard when i need to is no problem whatsoever.
I have two 270x in CFX. Two 270s are a great Crossfire setup. You get the performance of a 290, but by spending less. It would be a waste to sell the 270 and buy, say a 280x or a 780, when you can just get another 270 and get performance that easily beats one 280x or 780. But it is your build.