Bill, <<Hey guys, I've got a 6100/16 running 7.5.5. Do you think more memory will give me any improvement in either speed or stability?>>
Long time no see!
Increasing your physical RAM may well improve your system's speed and stability, especially if you are using large files or running multiple programs (or internet stuff).
Another thing I suggest is making absolutely sure you have all the VERY latest versions of your software. I've found that almost every time my system crashes now, it is due to a program error (NOT a system error). Often, when I upgrade (even if upgrading to beta's in the case of established programs), these crashes will disappear.
Another thing is to make SURE you have the PowerPC interrupt extension and WDEF Leopard. These seem to not only improve stability dramatically, but they also actually seemed to speed it up a tiny bit.