Todd,
For this format of discussion, i.e. all messages from a thread are on one page (I think it's called linear threading or somefing), the UBB and Discus
http://www.chem.hope.edu/discus/ are pretty much the best available. I *think* that the free version of Discus has more options, but the UBB is possibly prettier. I personally can't decide between the two. So I don't use either.
The one I find very good is WebBBS by Darryl Burgdorf -
http://awsd.com/scripts/webbbs/ , but it isn't this style of board, which creates the HTML pages, it's entirely data-driven. The upside of this is that it can be faster and it doesn't take up so much space on your server. The downside is that search engine won't spider the threads, and because you're running a program when a user is reading *and* writing, you run the chance of bogging the server down and having your administrator roaring in your earhole to shut it down.
That said, I like it and I reckon that if you're getting so many visitors that you're slowing things down, you should be getting enough back out of advertising to sort it out. It can be configured a number of ways, both by the administrator and the user. However, to get the "proper" administration tool for moderating the board through a web-based interface you have to register it. You can do it through telnet or ftp quite easily, but not *as* easily.
Also, customisation can be a bit of a bugger if you want to change more than the text shown on screen. If you want to change the actual design of it, you would want to be quite comfortable with Perl and have a lot of patience. But it can be done, here's one I prepared earlier -
http://corkcityfc.com/shed/forum/ (sorry about the language, they're soccer fans, what can I say?

).
Cheers,
adam