Huh, it does say 300W electric current. Ignoring that they mean power rather than current, I think that's probably the maximum power required if you load up all the slots. They're probably just saying 300W to give an easy answer if anyone has any problems with the motherboard. 200W should be fine for motherboard+ram+cpu+basic vga.

It's also about the quality of the power supply. You can get a decent 300W supply and it'll be more stable than a cheap 750W with ripple that takes it out of spec.

Exactly. Consider that a lot of less scrupulous PSU vendors quote peak power, not max sustained power when specifying their PSU. You'll be lucky if a Q-Tec (or similar) "300W" PSU manages 200W. Also, the really relevant bit for motherboard and components on it in this era was 5V. Different PSUs deliver different max currents on the 5V line. Here again, they probably conservatively went with the power rating where even the most feeble PSU could deliver the 5V current needed. A good, 5V heavy, 200W PSU would probably be absolutely fine, even with a Thunderbird 1400 and big AGP/PCI loadout.