Thankz @dek from fileconnect.net!
We have found some patches to optimize Quake3 v1.27-1.31 for your Athlon Thunderbird or Athlon XP. From what I've seen it will increase your FPS from between 5% to 45% depending on your setup. Includes SSE support.
Download: Thunderbird and Athlon XP downloads
News source: Fileconnect