I'm guessing since the GT is about 20W less than the GTS in both load and idle, they will be basically the same (~150W) in idle. But under load, the GT will be about 220-30W, while the X1950 Pro is only about 200W.
Either way it's not a huge difference, unless you are running 10 USB devices, a slew of HDs, and is seriously starved for power.