You have a quad core with SLi. That's a lot of juice.

I mean, yeah, you could run it with 450 watts, but your cards probably can't run at peak performance because there isn't enough power to support it. You'd be perfectly safe with a 650 watt supply in that computer, but if you want to upgrade the graphics cards down the road you might want to go higher. Besides, the difference in price between a 650W supply and an 800W supply is only about 40 bucks.

I have a 1.5kW power supply in this machine. Now that's overkill.