That's going to be situational. If money was a concern even in a slight degree, I'd probably wait for the time being or perhaps consider a different upgrade than GPU. Otherwise, if you go for the GPU swap, it's going to be very situational, depending largely on how much you get for the old cards and how much you need to pay for the new one, either of which, or both, could go wrong and set you back a hundred bucks or something compared to your initial estimates. And ultimately it's all about bang for the buck anyway.
If your mobo supports only Crossfire but not SLI, then for obvious reasons you might want to at least take a look at the new 400-series cards (Polaris/Vega) before you decide to switch over to nVidia.