While my setup may not be that good a match, I would make a couple of points...
Running MS Office under Wine might be a good choice, it's apparently noticeably faster than under Windows :-)
I use VirtualBox to run Windows, and the home directories are shared with linux - ie ~/Documents and "My Documents" are the same directory on the host hard disk.
Both Firefox and Thunderbird are configured to have the same local storage in linux and windows, so it doesn't matter which they're fired up under (and I think they can be fired up under both at the same time - that's certainly true of firefox), you see all your mail, bookmarks etc in either.
And I use linux as the host os because I use xdm so both my wife and I can share the same pc at the same time :-)