Anyhow, yes, you can boot XP from an eSATA drive. Many notebooks only have eSATA drives. My desktop boots from an eSATA drive, and I did not need to do anything special to make that happen.
It all depends on what type of SATA controller you have and how the BIOS handles the SATA drives.
The general demarcation is if the SATA controller is part of the motherboard chip-set, or a separate controller.
If the SATA controller is part of the chip-set, you should have no problem installing XP to a boot drive connected to a SATA port. You do need to verify that the SATA controller is in the boot sequence, and that the correct drive is set as the boot drive. You may not even need to install drivers.
If the SATA controller is on a separate controller chip or card, then you do at a minimum need to install the driver. For some of the on-board SATA controller chips, you need to enter the controller BIOS (after the normal POST) and configure the drive as a RAID volume. Even for just a single drive.