I worked on a Mac Pro once that someone had 64GB of RAM installed in... took the better part of a minute to get the startup gong.
They ended up splitting the memory between a few machines as it became obvious that the applications being used (Adobe CS stuff) wasn't going to use all that RAM in their use case, and other machines needed it more.
Although the new Final Cut Pro X eats RAM for breakfast. Our editing Mac Pro at work regularly uses 16GB+ when working in FCPX with multiple projects and events. Frustrating at times, to say the least.
They ended up splitting the memory between a few machines as it became obvious that the applications being used (Adobe CS stuff) wasn't going to use all that RAM in their use case, and other machines needed it more.