Maybe not now but for 300$ more you can get almost as twice as fast computer which will also make the resale value noticeably better and you never know what future brings. I think it's worth it, even though even C2D might be too powerful for her
If she does any DVD encoding using something like Handbrake, then the quad-core will be most advantageous. Really, for the small price diff its a no-brainer.
Get the quad, for a small extra price it's far more future proof, has better resale value and as previously said software like Handbrake and Toast will scream on it as they use every core available.