Everytime someone type out crossfire as “Xfire” I think of this: https://xfire.com/
Mainly because That’s an “X”, not a cross.
Well, †-fire has some unfortunate implications…
Pretty much everything that needs to be said has already been said 
But still here are my two cents:
The GPU you could get with that budget would probablt mean GTX680 or HD7970. But as stated by others this would mean the rest of the system would become a huge bottle neck. So yes upgrade CPU and mobo as well.
Just for the GPU decision: I would go with the 680. in my country they are about the same price and it preforms just a little bit better. from what i’ve seen in reviews this generation of nvidia GPU’s are looking more efficient compared to AMD’s architecture. Nvidia gets more performance with less transistors (and by extension smaller die size) so in my book they win this round.
Nvidia didn’t even pop out their high end gpu. The gtx680 was supposed to be a gtx670ti. The real deal will be the GK110 chip which will possibly be released in october. Seeing how the 680 is flattening the 7970 already AMD doesn’t stand a chance.
And to think I wasted my money on a 6950. 
I guess I’ll let it run its course and switch back to nVidia when the time comes.
God damn it, AMD, I thought you were supposed to have better bang for buck ratio! 