Think of the 7900GTX as the 90nm version of the 7800GTX 512MB. This allows it to get such high clocks as 650 - 700+mhz on air without using a volt mod like the 512 did. The G71 was already in development while the X1900s were and ATI thought outside of just raw power and changed the shader architecture. Nvidia just stuck with: faster core speed = killer card but ATI saw: more efficiency = killer card.
And because the X1900XTX is so dam efficient with the shaders it beats out the 7800GTX 512MB and also the 7900s.
The 7900GTX is only 8-10% faster then 7800GTX 512MB.
I wouldn't call the 7900GT a "beefy" 7800GTX because it's only 20mhz core and 50mhz memory (100mhz effective) faster then the reference models of the 7800GTX. Think of the 7900GT as the 6800GS for the 7800GTX.
This also means that the X1900XT/X might be more powerful then the Playstation 3's GPU since it's also using the G71. It seems these next-gen consoles are already being beat out by PC GPUs but of course the cell in the PS3 is more powerful then many CPUs out there. Except the X360's GPU is more powerful then any card out there.