Cool.
Since you already have Vista on your sata drive we're gonna go about this bass ackwards, but let's dive in!
Unplug your Vista drive and install XP on the PATA.
Now, after you plug that Vista drive back in, you have to decide how you want to configure your dual boot machine.
You could just go into BIOS each time you want to start up your computer to choose which drive to boot. That would be the simplest solution.
However, If you want the "standard" dual boot machine, you will need to install the boot loader on your computer.
If you choose a standard dual-boot setup you can repair Vista's boot loader by booting to the Vista DVD after XP's been installed and instead of installing choose the Repair my computer option in the bottom left corner of the second screen. Start up Repair will then restore the new windows boot loader and make both Vista and XP bootable.
or
After you install XP, You can boot to the XP drive and follow the instructions here
http://support.microsoft.com/kb/919529/en-usto enable a dual boot without having to access the BIOS every time you start up.
good luck.