1000w

Hi, I want to purchase a new ups.... My system has c2d e4300 overclocked to 3.0 Ghz Mb Asus p5n-e sli n i got a 8600 gt Graphic card... May be in the near future i may add one more graphic card to it... So should i purchase a 800 W ups or should i go for 1000w ups as I heard 1000w Provides...