Generally, the more components your computer system has in it, the more power you need to power these components. Furthermore, higher performance components tend to require more power. For example, an 8800 Ultra graphics card will need more power to function to it's full potential than an 8800 GTS graphics card. A good rule of thumb is to use the power supply recommendation found on the box for your graphics card since the graphics card uses a good percentage of your system's overall power consumption and the recommendations are made based on fairly high performance machines. Naturally, one should take into consideration a little bit of overhead if their system has multiple processors or several hard drives in a RAID array.