Noob question about tubes and tires

Ok so I'm not new to cycling, I'm a daily commuter, but I'm clueless when it comes working on my bike. In an attempt to remedy this I've decided to build up a 29er, because I want a more trail capable bike. I know this may not have been the best idea, but I'm too far in to turn back now.

Anyway on to my question, I think I'm going to pick up some budget friendly tires from nashbar to put on the bike until I figure out which set of tires I really want. I was looking at the Kenda Keen commuter tires so I can atleast use my new bike on the streets after I get it build up. The Kenda Keens are a 700x45 which by my math would be the equivalant of a 29x1.77. So since I have yet to find 700 tubes that are rated for anything higher than 38mm I figure I should use a 29er tube for these tires. At nashbar they have Fuji 29x1.75 tubes, but they are described as a 48mm MTB tube for 29x1.75 tires. Will these tubes and tires work together? I've read up tube sizes a bit and from what I gather using and undersized tube will work, but it will be weaker, where as using an oversized tube isn't such a good idea. So I was wondering if a 48mm tube will work with a 45mm tire?