Hi,
The exact Wattage requirement varies from system to system, and if we plan to Overclock the GPU/CPU, then the PSU should have some headroom as well. That does not mean the GPU will always draw 750 Watts, assuming you have a 750 Watt PSU.
This is actually the recommended PSU for the whole system, and not just the GPU alone, to be on a safer side. The system will still use/pull only the maximum wattage required, regardless of whether you have 500 or 850 Watts PSU.
If suppose you have a 500W PSU installed on any RIG, then that does not mean that it will always draw the full wattage/500 W. Nope.. The amount of power drawn is determined by the number of components on that rig/computer, and how much they actually require. The amount of power drawn will only be equal to what is required, and not more.
Suppose, if all the components of a rig require 400 W to run, then the "load" on the PSU will be 400 W, and hence the power draw of that particular PC will always be 400 W (depending on the efficiency obviously), regardless of whether a 500 Watt or a 1000 W PSU is installed. PSU efficiency is a different matter though, as how well the PSU converts the AC power it receives from the outlet, to DC. Any electricity which is not converted from AC to DC, is given off as heat.
A PLATINUM certified PSU might help you save a little on your electricity bill, over a year, though this depends on many other factors as well. Because any high quality Tier 1 platinum certified PSU is rated for at least 90% efficiency at 20% load, 92% at 50% load, and 89% at 100% load (just a rough estimate).
Edit:
Btw, what's the current make and Model number of your PSU which you are using now ?