This sounds exactly what I just completed to get my system dual booting - the only difference is I use Windoze 2000 rather than XP. The procedures should be the same since XP is just an enhanced(?) version of 2000.
I used the HOWTO at:
My system was originally running Windows 2000 on a 30 GB hard disk configured as the Primary Master. I added a 10GB disk as Secondary Master. Linux sees these as hda and hdb respectively. I let the Linux install (Red Hat 7.3) partition the 10GB disk automatically, then installed GRUB to the boot sector of hdb1.
If you follow the HOWTO you shouldn't have any problems.