"what happens to grub when I install Core 2?"
There are several possibilities that you can chose from. The key is that there can only be one bootloader on the MBR.
1. When you install Core 2 tell the installer not to install a boot loader. Then edit the Core 1 /boot/grub/grub.conf file to include both Core 1 and Core 2.
2. When you install Core 2 tell the installer to install a boot loader on the MBR. Then edit the Core 2 /boot/grub/grub.conf file to include both Core 1 and Core 2.
3. You can remove the Windows XP grub entry at any time whether Windows XP is physically present or not.
4. You can add the Core 2 grub entry at any time whether or not Core 2 is physically present or not.
5. You can have the Core 1, Core 2, and Windows XP entries in /boot/grub/grub.conf
at the same time even though all three are not physically present at the same time.
Here is a grub tutorial to tell you how to edit /boot/grub/grub.conf
Be prepared. Create a LifeBoat CD.