My original BIOS dated sometime in 2004 had a 'Command Rate' setting of 1T. When I upgraded the BIOS to the March 6, 2005 version, its default was 2T.
So, recently to experiment I changed it back to 1T. It worked fine, and it made a minor performance increase in memory as measured by PCPitstop.
Then after a day I began to have all sorts of crashes in various programs, and finally I got a boot message saying it could not load the OS (XP Pro SP2).
Got back in to the BIOS and loaded optimized defaults, and then restored a True Image disc image.
All back to normal, with the default Command Rate back to 2T.