Hi Geezer207,
The graphics card needs a 500W power supply as per the recommendations. This wattage includes other system components as well. The graphics card alone does not require 500W. You can use the Radeon HD 7870 video card with 460W power supply on the XPS 8500. However, if you there is any lag or freezing during gaming, you will need to upgrade to a 500W or higher power supply
Please let me know if you need further assistance.