How important is power supply rating when it comes to upgrading video cards?

i own a pc with a power supply rated at 250 watts however every Replacement graphics card ive looked at lists a power supply of 300 watts in the minimum requirements. Do I really need a new power supply just for the 50 watt difference?|||It MIGHT work. MIGHT. If it doesn't, don't quote me, and yes it is very important. I'd invest in a more powerful power supply.