Regarding nuclear EMP effects.
I was in the process of writing some huge, long thing about the known effects of EMP from nuclear explosions, with a list of references and all that stuff, but somebody else already did a better job, so you should go look at these instead:
A high yield is not necessarily required to cause a lot of damage. The USSR's K-project tests in 19621 and 1962 didn't have an especially high yield; the most damaging shot, test 184 (K-3), only had a yield of 300kt, and it hosed a lot of shallow-buried cables and set a power plant on fire. (The explosion happened at an altitude of 290km.)
The strength of the pulse depends on the strength of the Earth's magnetic field in the vicinity of the explosion and the yield of the bomb. Gamma rays from the bomb produce high-energy electrons through Compton scattering in the atmosphere, and these go spiralling around magnetic field lines; in doing so, they lose energy in the form of electromagnetic radiation. The Starfish Prime shot near Hawaii was not in a very strong magnetic field region; hence, despite its 1.4 megaton yield, it only produced a maximum electric field on Hawaii of about 5.6kV/m, which was sufficient to screw up parts of the phone system and to knock out a bunch of traffic lights (although only a small fraction of them).
Remember, however, that this is the stuff of nuclear war, and nothing less. If somebody's shooting off nuclear warheads into the airspace of other countries, you might have a lot more to worry about than whether your line conditioner is stout enough to keep your mail server running. Of course, civilian infrastructure would be at greatest risk of EMP damage. Military installations are built with this stuff in mind; civilian infrastructure damaged by EMP can be repaired later, so long as someone does a good enough job of fighting the war that something is left to repair later. Hopefully, that isn't necessary.
Further reading, for those interested:
Military Research and Development Subcommittee of the Committee on Armed Services, US House of Representatives. Electromagnetic Pulse Threats to US Military and Civilian Infrastructure. Hearing. One hundred sixth congress, first session, 1999.10.07. Accessible via http://commdocs.house.gov/committees/security/has280010.000/has280010_0.htm
Seiler, Louis W. Jr: A Computational Model for High Altitude EMP. Air Force Institute of Technology, 1975. Accessible via http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=A009208&Location=U2&doc=GetTRDoc.pdf
Vissitoe, Charles N. Did High Altitude EMP Cause the Hawaiian Streetlight Incident? Sandia National Laboratories, Electromagnetic Applications Division. 1989. Accessible via http://www.ece.unm.edu/summa/notes/SDAN/0031.pdf