Performance wise the 7990 is better than the 780. However, the 7990 is loud and is way too hot to work properly (you may need liquid cooling to make a 7990 work properly). Also the 7990 is two 7970 video cards fused into 1. This means that you have to deal with crossfire issues like microstutter. 7990 requires a 750W power supply. GTX 780 requires a 600W power supply.
I recommend the GTX780 if you're thinking of buying.
Performance wise the 7990 is better than the 780. However, the 7990 is loud and is way too hot to work properly (you may need liquid cooling to make a 7990 work properly). Also the 7990 is two 7970 video cards fused into 1. This means that you have to deal with crossfire issues like microstutter. 7990 requires a 750W power supply. GTX 780 requires a 600W power supply.
I recommend the GTX780 if you're thinking of buying.
I would wait for the 290X. A titan killer for $600 c'mon? don't know if its more powerful than 2 7970's but its damn up there. AMD's new drivers are excellent with CF systems. I would know. \/