OK I read the answers and I think that they all missed answering the questions. Since I was on the same mission (to scan medium and large format black and white negatives), and found a solution, see if these thoughts help you at all. I hope this doesn;t come across as though I know all the answers...I don;t. But I do have some experience in doing this....
WHAT I USE TO SCAN BLACK AND WHITE
I use a UMAX POWERLOOK 2100 XL. It has a maximum optical resolution of 800 dpi but I bought it very cheaply for having a 13'' x 19'' scan bed and a full sized transparency adaptor. They now make the scanner with higher resolution but read below about that before you get super excited.
I had trouble scanning black and white negatives or transparencies on a number of different scanners. Assorted problems....perhaps the silver content (reflective metal particles) in the Black and White film and prints were a problem (as compared to the dyes used in color film). The Powerlook comes with software that automatically adjusts exposures for the black and white (I think it is called Binuscan) and it really works. The same scanner without it won't do the transparencies properly. I successfully scan all sizes of black and white negatives including large format.
Part of your challenge is finding a scanner that has a setup that allows you to scan a variety of larger format negatives. This isn't common in the lower priced scanners...in fact rare in even the moderate priced ones. At least non-existant in all the ones at Office Max, Staples, Circuit City etc. And I have no idea if any of them really handle black and white well.....get a recommendation before you buy one for that purpose or you may be sorry. There should be presets for scanning these types of negatives or you may end up out of luck, no matter what they tell you.
RESOLUTION YOU WILL NEED
Depends on how you will use the file. Depends also on how much larger you want to make the print.
For offset printing:
Generally, it is accepted that 300 DPI makes a very good 150 line screen offset printing photo. So if you want an offset print photo two times bigger that the negative in each direction, scan it at 600 DPI to end up at 300 dpi in the enlarged size for a 150 line screen print.
For computer printing:
The more true resolution you can get, the better. Of course there is a limit, but with printers printing at such high resolutions, more dots making up the image are generally good. But you should go to scantips.com for information on scanning. They show you that photo paper has a resolution of less than 600 DPI. Scanning at resolutions higher than that gets you more dots but not more detail. Which means that scanning at higher resolutuions increases your file size but does not impact sharpness. Since there is always that careful balance of file size vs effective resolution, this is critical to understand. Old black and white prints and all negatives have higher resolution (and lower grain) than print paper, so you can scan at higher resolution, but even they have limits. I don't know what it is for black and white negatives, so I just scan at the optical maximum for my scanner.
OK have I bored everyone enough? If you want to talk more about this, drop me an email to firstname.lastname@example.org with Photo Questions in the message line with your phone number and I will call you (hey free weekends!)