nope, you can't upgrade to the server product (Windows 2003) from XP.
installing 2003 will modify the mbr of hard drive and the boot.ini file.
if you go ahead and install 2003 on your second hard drive, you might be able to dual boot. i do not know if 2003 uses the same ntldr and ntdetect as xp. i would shoot ms an email asking them maybe. what i think i would do to test this is disconnect my xp boot drive (to give me courage and force the mbr to be modified only on the second hd) then install 2003 then attempt to modify the boot.ini file so it will give you a choice of os's at boot time.
i have never tried this so i don't know if it will, the ntdetect and ntldr files need to be the same i think...and i'd backup my boot.ini (on a cd or floppy) file first and make sure i had my xp install disk on hand in case you need the recovery console to fix things back up...
the boot.ini file, as you probably know, is a hiddenm system, read-only file that lives at the root level of your xp os partition (ie: c:\boot.ini)
so you'll need to use the attrib command to remove those attributes so you can edit it.
i think the worst that can happen is the you won't be able to boot both drives without a third party boot manager. you may have to repair the xp install with recovery console tools like fixboot or fixmbr. they work and are good for you to know about. so i would play with those before i started...