Depends on exactly how you want Ubuntu and WinXP to be selected on
boot up. A standard dual-boot setup will be created automatically by
installing Ubuntu and when it asks for where you want it to install
just select the second drive.

If you want to install Ubuntu to the second drive without affecting
the WinXP drive in any way at all - i.e. you don't get a boot option
to select which one you want - then that's a bit harder and means some
work on your part to manually change the boot drive every time you
want to change the boot OS. If you have a BIOS that has the [F12]
option to change boot drive then this will be easier than if you have
to go into BIOS every time to change boot drive order.

BTW - if going the totally separate drive route it's advisable to
disconnect the WinXP drive during the Ubuntu install. If it doesn't
see WinXP at all it will make the install easier !!