Congrats on raising funds for your latest trip - I know it's no easy task in this economy. As for the mystery number on the batteries, it indicates the capacity of the battery.
Here's how it works, your device, the camera needs a certain amount of battery power to operate - that's the volts, the 7.4 I believe you said. The second number, the capacity of the battery indicates the total capacity, basically how long the battery will last as it is giving juice to the camera. The bigger the number, the longer the time. I will warn you that these batteries are not like digital still camera batteries - a battery + 1 backup won't cut it. I figure that my camera uses a whole 7200 battery for each hour of film, so if you'll be traveling to a remote are, you are going to want a lot of batteries. They also don't charge as fast as digital still batteries because of their increased capacity. I would say that yes, the 7200 is worth the extra price - I believe there in the $140 range at B&H. The only downside to the larger batteries is weight, the 7200 weighs a couple pounds I believe - but lasts almost twice as long as the smaller capacity.
One more thing I should mention, the FAA changed their rules about Li-ion batteries, and now have very strict limits on the number, and capacity of batteries both in carry on and checked luggage, so if you're flying, you might just check to make sure you'll meet their standards.
Best of luck, I think it's a great project you're working on.