I'm gonna put a new dunlop k555 on the back, same size as stock.  What size tube do I need, and how do I select it?  

I noticed that this tire is a tubeless tire also.  What is the difference?  I assume it will work with a tube, right.

You want the tube to be 5.5" by 15" and, if you have to, always go too small on tubes rather than too big, the smaller tube will expand the bigger one will have wrinkles and won't be safe. Most tubes are sold in ranges of width, example 5.00/5.50 x 15". Good luck and make sure you get the right kind of valve stem!

edit: I always get the straight threaded type of valve stem, just because I think they are the easiest to install.

If you are keeping the stock wire spoke wheels, no matter what tire you get you will have to run a tube. Tubeless just means that it will work on wheels that do not require a tube.

Here's a thought, or a question, or something...

If you run a tube in a tubless tire with a wire-spoke wheel that is not sealed up around the spokes, what happens when the tube suddenly bursts?  Does the tube blow out, but the tire then maintain some of its integrity while the air from the tube leaks out around the spokes?  

I've been reading on this and it seems that the big problem with our tires is that they go flat instantly when the tube blows, causing the rider significant distress.  Will the tubeless tire go flat slowly enough to allow some measure of control while you get the bike shut down?

http://www.bikebandit.com/tubes/c/a642338a639923a640300

That one should work.