What will happen if you supply a 450W graphics card (two pin plugs) with only 400W, one plug? It is working.?

I recently purchased and installed a sapphire HD 3850 (AGP) video card, i installed it and i have so far been able to play my games at full blast with no noticeable problems. However the requirements on the box include a 450 watt power supply on your computer and two pin power leads to plug into the card, i have however only 400 watt and one pin. Is this a significant issue? (computer has reset itself twice without prompt during a in game menu).