GraphicsEnthusiast :
r9 280x crossfire would be better. The sapphire r9 280 toxic edition beats the 780. So two of them would be even better.
Not sure where you get your info from. I agree that Xfire 280x's will be beat a single 780 easily but you actually believe even the highest overclocked 280x compares to a 780? I've owned a 7970 and overclocked it to about 1100mhz (not exactly toxic but somewhat close). The benchmarks and in pure performance the 7970 didn't stand anywhere near where my EVGA GTX 780 Classified has shown (granted one of the highest overclocked 780's). On Unigine Heaven I only got up to about 35fps on average OC'ed whereas my 780 averaged around 55-57fps stock. Firestrike score? OC'ed: 6000+. Stock (Classy): Nearly 10,000. They don't compare very well.