If you have XP + OSX dual boot setup, you can easily upgrade it into a Vista + OSX dual boot setup with a tool call EasyBCD, and use Vista's bootloader to dual boot.
1. Install Windows Vista on the partition where XP is currently installed. You can format your partition during Vista installation to get rid of XP.
2. Install EasyBCD and run it.
3. Under "EasyBCD Toolbox" click on "Add/Remove entries".
4. Under "Add an Entry", click on the "Mac OS X" tap.
5. Choose "Generic x86 PC" as the Platform, then name the entry you want it to show in the bootloader.
6. Choose "Auto-configure Mac settings".
7. Finally, click "Add Entry", then click the "Save" button.
8. Now you can click on "Configure Boot" under "EasyBCD Toolbox" to make it boot the way you want. You can choose the default OS, and the timeout for the menu to stay on screen. Remember to click "Save Settings" afte you are done.
9. To complete, just reboot.