Honestly, it's hard to say. As Akuta said in the other answer, yeast viability is decreased by increased temperature as well as age. Further, you don't know how old your yeast is until it arrives. Northern Brewer definitely SHOULD be moving enough yeast to send fresh stuff, but it doesn't require a lot of age to decrease viability.
Here's what I recommend: Use Jamil's Yeast Pitch Calculator (http://www.mrmalty.com) and calculate how much yeast you'll need for your beer, with a starter, including dates. (You should always do a starter, whether the yeast is fresh and arrived cold or not.) Tell the calculator that your yeast is a month old, even though NB's yeast will likely be fresher. That will help account for the reduction in viability you may experience due to heat. A great many homebrewers are surprised at how much yeast they really should be using, even though Wyeast and White Labs say on their packaging that their packages contain enough yeast for 5 gallons of wort. That's really dependent on the gravity of the wort, as well as the age and handling of the yeast.
Depending on the OG of your beer, I suspect you'll find that you may want to consider purchasing 2 packs/vials of yeast instead of just one. Jamil Zainasheff has done the yeast research and his pitch rate calculator is absolutely indispensable, and can help you account for things like temperature, somewhat, and age.