its just not a good idea to run a PC on the edge of what power it needs to run properly. You may decide to run it on a 300 Watt PSU anyway and have no issues and think "Ha what did they know" OR it could fail when asked to deliver more than its really capable of and fry your whole computer. Extreme and unlikely but possible.
When the makers sell a PSU its rated in Watts. Your Graphics card doesnt work in Watts its more concerned with how many Amps are on its +12V rail. Depending on the make model of PSU there may be 1-2 or more of these rails.
The real issue is here because, and there really is no way to sugar coat this
some makers lie about the actual available Amperage. When i say lie i guess provide misleading information as to the Amperage available would be more legally correct.
With that in mind what is the make and model of your PSU and what does it say on the information plate as far as all the rails are concerned ?
Mactronix