Modules and performance
It depends on the module.

You can create a module which has a simple info file and a blank module file, compare that to views which has loads of file and implement lots of hooks and create it's own as well. Obviously this requires more memory than a blank module.

Performance depends on a lot of factors. A module can use a lot of memory but not slow the side down much while the reversed can be true as well. Usually the bottleneck is r/w to the database. A single bad query can take seconds to perform, most tasks in code is not that slow. Iterating over a lot of objects and munching data can, however, take a lot of time.

So it's hard to say how a module will effect performance, but it's always a good idea to limit the amount of modules used when possible. I haven't heard of a thumb rule though and to be able to make one, you would need to know the hosting environment.