Though you can install the os's in any order, its usually easier to install the older one first (xp) and then the newest one, that is because you want to use the boot loader from the newest one, and if you install the oldest second, you will have to redo the master boot record with the new os.
Anyhow you will probably want to use EasyBCD to fix your boot manager so that it also shows xp.
http://neosmart.net/dl.php?id=1
http://neosmart.net/wiki/display/EBC...B694709DE4F5F5
EasyBCD works fine with windows 7
Lastly don't forget to move your boot.ini file from windows xp to your windows 7 root directory. You will also probably have to edit it some so you don't get two boot menus.
Otherwise what you are doing is the same thing as a vista & xp dual boot setup, so any instructions are the same. This is what I am currently running (though on 2 partitions on one sata drive). As far as I am aware there are no issues with using mutiple drives with SATA, just make sure your version of xp properly supports SATA otherwise you may have to make a custom install disk and slipstream the SATA drivers into it.