Jeremy,
The PowerBook 520 doesn't have an FPU and the 68LC040 in them is buggy, so
floating point emulation doesn't work.  (at least last I tried on mine) so
NetBSD doesn't really run.  The linux folks seem to indicate that it works
for them though I haven't tried...

The PowerBook 540c does have an FPU so NetBSD ought to run fine.

Both of them are pretty reasonable in terms of performance.  If you go with
MacOS I would stick to 7.6, 8.0 is a little slow and takes up lots of disk.
I had a 520 all through college '94-'98 and it worked great even running
Netscape 4.7x.

The other concern would be disk space since mine at least only had a 150mb
HD.

Hope that helps.
--brendan