4× if used on Magnemite, Grimer, or Tangela (GSC)
4× if used on a Pokémon with a base Speed of at least 100 (HGSS)
The Fast Ball causes some controversy as the game description of the ball tells you it will work on Pokemon that can flee from battle. However, in the game's coding it only works on three Pokemon - Magnemite, Tangela and Grimer.
In HGSS the Fast Ball will be 4x capture rate if the capture target has a base speed stat of over 100.
Hope I helped. :)
Source and Jaja's answer. :3