Nice one, I seem to have become a dumping ground for 5x0 Powerbooks, having been given a nice 520 greyscale and 520c most recently - they've both gone to good homes (iMac600 one of them!).
One of my previous posts here:
http://68kmla.org/forums/viewtopic.php?t=951&highlight=
... I was lucky enough to score a whopping 167Mhz Powerbook 540c with 36MB RAM in that auction
Years ago, 68KMLA member Flash! was kind enough to give he his 540c with Apple 100Mhz upgrade, and both machines are running Marathon via a serial connection quite nicely - cool to see them both hammering away at a nice frame rate 
JB
One of my previous posts here:
http://68kmla.org/forums/viewtopic.php?t=951&highlight=
... I was lucky enough to score a whopping 167Mhz Powerbook 540c with 36MB RAM in that auction
Years ago, 68KMLA member Flash! was kind enough to give he his 540c with Apple 100Mhz upgrade, and both machines are running Marathon via a serial connection quite nicely - cool to see them both hammering away at a nice frame rate JB