R9 295X2 8GB Easily, that is two R9 290X 4GB's on the same PCB!, that'll destroy any of those cards but has a whopping price tag.
You can wait for the GTX 800 series, and may be a good option, but no single GPU'd card released (GTX 880) will be the dual GPU'd R9 295X2, for obvious reasons, 1 better GPU vs 2 slightly worse GPU's.