I think you're confused about the wattage recommendation. Very few cards require 300W & the ones that do are on the extreme high end. When a graphics manufacturer recommends a minimum PSU wattage, it's for the enter system with the video card installed, not for the video card alone. The manufacturers realize that not all PSUs are created equal so they play it safe & over-recommend the wattage.
Please post your complete system specs, including the power supply specs (not just wattage), & the make/model of the video card you plan on getting.
In the meantime, here's a chart that should give you an idea of the power required for just the video card:
If your card isn't on the list, just do a google search using the make/model of your card along with the terms "power consumption" or "power requirements".