Are you talking about BIOS update or BIOS chip replacement ?
In your case, I believe you do not need BIOS chip replacement. But you may need a BIOS update. Besides, BIOS chip replacement ( an upgrade in this case ) may not be possible.
On the other hand, BIOS updates are done on the same chip. You download the BIOS update from the manufacturer's site, Asus in this case, and use a method ( USB flash disk or diskette drive or Asus Updater ) to update BIOS.
For further information, I suggest you to read your motherboard documentation and http://www.largeharddrivesupport.windowsreinstall.com/winxp.htm