It depends on the river(s) you want to float, how many folks are in your party, and how much equipment you want to take. There are many different kinds of rafts and the common advice is to let the river choose your raft. If you look further down the Alaska thread, you can find pictures of the raft I use. It is a 16 foot SOTAR Radical. I have had mine loaded with 2 mid size bull moose, 2 people and all related gear. I was on a class I to II+ river. While there are many different good brands, NRS, Aire, and SOTAR seem to be the most mentioned/used up here. I would recommend reading a few books on float hunting: Hunting Alaska's Wild Rivers by Michael Strahan (owns the Alaska Outdoors Forums) and A Complete Guide to Float Hunting Alaska by Larry Bartlett (owns the Pristine Ventures website/forum). Larry also has videos out of float hunting that are great as well and sells great rafts as well on his site. On the forums both will answer your questions and are recognized experts on alaska float hung.
You can/will get several different answers to your questions about rafts. I would recommend checking out both websites as there is a wealth of information.
Good luck and feel free to ask questions. I'll try to help out as best as I can, though I am getting ready for this years float hunt. I leave in 3.5 weeks or so.
V/R
Garion33