SATA ACHI questions

I am putting together my first machine: Asus P5B, 2 500 GB HD(both are SATA II), sapphire x1950xt, GSkill 800 (2GB), Corsair 520 PS

It booted!!

I now have to set up the bios and have some questions:

I am going to install XP SP2, I have to set up the SATA drives do I enable ACHI or leave it as IDE (I don't want to do RAID). If I do enable ACHIwhy am I doing it? If I enable it and want to disable it later am I in trouble??

Choices are ACHI, IDE or RAID (ACHI and raid require me to add drivers with Win XP install)

In the old IDE world, the boot drive was identified and setup with the format command - with two SATA drives how do I identify which one is the boot drive and the how do I partition and format each drive? (I cerrtainly don't need a 500 MB Drive C:!

Any and all help appreciated

Steve