Configuring My Documents

Hi,

I'd like to know the pros/cons of the folloiwing scenario:

PC1 - XP x64 SP2
PC2 - Vista Ultimate (yet to be built)
Both computers will be in a workgroup, and will have the same usernames and
passwords (2 users).

I'm thinking of purchasing an external hard drive with an Ethernet
connection. Would it be possible (or worth it) to redirect the My Documents
folder for the users on both machines to a share on the network drive, so
that either user can access My Documents from either computer? Would there
be authentication issues? Or would mounting volumes be a possibility?

Thanks,
Mike