Those who work in the Unix field don't usually benefit from Microsoft certification unless they work at an employer...
that runs a hybrid Unix-Windows environment. In fact, it's not that uncommon to find companies or organizations that use Unix servers but still run Windows on the desktop. In the purest sense, possession of Microsoft knowledge and skills is not as important for those who want to stay on the Unix side of the street, but in terms of finding work, a combination of the two will certainly increase your options -- although it may not increase your pay.
If you already know something about networking, Network+ is pretty elementary. On the other hand, MCSE is both pretty challenging and on its way to being replaced next year by new Windows Vista-based credentials. MCSE won't be passé for at least three more years after that, or around 2010. If you wanted to do more networking stuff, I'd advise pursuing the Novell CLE or perhaps the RHCT/RHCE credentials or perhaps even the Sun Solaris admin stuff rather than go down the Microsoft trail, unless you're convinced you want to straddle both sides of that divide.
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