As stated, it won't work well. XP won't have the necessary drivers to work on the new motherboard and will likely BSOD. You might be able to get away with a repair install, but it will be best to start from scratch.
Also, if you really need to dual boot, the older OS always needs installed first, so you would need to install XP first, then Win 7. There are ways around this, but it adds extra work.
Sometimes it's best not to have a dual boot menu and just use the bios to switch boot drives. In this case, make sure only one drive is attached at a time when installing each of the OS's. Then whenever you boot, hit F12 or whatever bios boot menu you have and select the disk to boot off of.