OK, here's my opinion.
Vista will be very different.
It is the newest OS, but she may find it hard to get used to.
XP Pro X64 Edition, she may find it faster on her machine, but it has major driver and compatibility issues.
So I wouldn't get that either.
I'd actually get XP Home, unless she has a reason to go fro XP Pro.
For most people, Home is fine, and cheaper.