Fits after a few minor mods to the G5 rear chasis frame (had to trim a small section from the rear side support for the heatsink to clear. I also swapped a single slot bracket panel from the rear of my 6800GT to fit it in the single slot.
So far it is running great - I can't believe how much lower my temps are with this cooler design - 38C Idle and 58C load! (GPU Core 550 / Memory 1700). The best part is this cooler is dead silent!
Power does not seem to be an issue - from my research the 7800GTX 512 power requirements are close to the 7800GTX 256.
Note: I am using a PC50 300W PSU in this Shuttle