This card isn't really worth it if it does 24MH/s using 150Watts. My 7950 uses 150Watts and hashes at 20MH/s.
The 7950 is Tahiti based @ 28nm, ~3 generations behind the rx 480 and about 4 years old (released in 2012).
Tahiti uses a lot of power at max load unless you undervolt or downclock.
The rx 480 is @ 14nm and power utilization benchmarks already indicate it uses significantly less power at max load.
Comparing the 480x to the Tonga (which is newer and more efficient than the Tahiti), it is at least 17% more power efficient in watts and hashes Ethereum ~20% faster.
And well the Tonga is 20-35% more power efficient than the latest Tahiti chips.
By this data, the rx 480 will likely be at least 20-55% more power efficient than the 7950 hashing Ethereum and hash around 20% faster than a 7950.
A less facile view of the problem:
1) As I pointed out in an earlier post, initially, RX480 will have thermal management problems:
1) In going from "28nm" fab technology to "14nm" fab technology, the the chip area has shrunk by about 1/2 to 1/4 of the area of a roughly similar 28nm fabbed device.
2) Say you have a 100W chip. If you shrink its area by half and everything else is the same, the amount of thermal energy per unit area that has to be dissipated has doubled. The problem is that you may have to use a technology such as water cooling to handle this increased heat dissipation per unit area. The heat transfer properties of passive radiators/conductors using metals in conjunction with thermal grease have their own limits in terms of how much heat flow they can sustain per unit area.
The thermal management issues will take months to cheaply work out. What shrinking the die does in this case is increase the heat flow per unit area and lower the thermal mass. So whatever system is controlling the chip temp has to be faster and heat dissipation is now a real issue because the thermal resistance of the chip-grease-cooler stack hasn't changed.
Most likely OC'ing this chip and not using the OEM software to do this will void the warranty. I would also void the warranty is the cooler is separated from the chip.
The 7950 owner is quite right in holding on to this card until these issues are settled.
Right now the older cards are useful in places where electricity is cheap and at least with these 1st RX cards air conditioning is free.
Finally, in winter I'm using the Phil Ma solution for my older cards and bringing them inside to provide heat.