I'm running the 2.0 quad mini now, for an office computer, I can't imagine what needs one would have to justify a faster machine.
If your office work includes encoding/decoding video files, RAW photos etc, then it may be needed, but for general office productivity even the 2.0 quad is total overkill.
What is really nice about the quad, is that it will better support the multitasking options in Lion better than the others. With it being so easy to set up multiple desktops (for me running on two monitors) with multiple running apps is really helpful. Using the quad helps reduce lag when switching between them.
More speed is always nice, but the 2.0 quad is pretty impressive already.