I am through with this since a few months. 2008 (32) with vista clients (64 and 32) with hp and its famous unidriver 6.
Install the printer drivers to xp locally as if the printer were connected to xp. afterwards, do just like before. This time, xp should be able to provide the server with an x86 driver that other XPs without having unidriver installed locally can use succesfully.
Good luck