Before you install xp64 make sure you back up your data.
Also before you can install xp64 you will need to download the 64bit drivers either ata or raid depending on what you want here
Copy these to a floppy and then during the beginning of the windows instalation it will ask you push F2 (i think F2) to install third party drivers.
From there it runs like a normal windows instalation. You will need to decide if you want to erase your original partition with your original install and set partition sizes and our done. THen you will install the 64bit lan, video, audio, ect. drivers off of sager site (although more up to date ones may be found on the manufactures site).
Hope that helps.
Edit: Sager has updated the 64bit drivers for the 9860 but not for the 4750