What is the impact of changing open files limit? To install Riak on OSX, Basho recommends to change the files limit on the system: Open File Limit.   

But before doing that, I want to know the impact of changing that on the system. Should I expect slowing the system? More RAM usage?
There is no impact. You're simply changing the max number of open file descriptors a process can have. Without it, any software that needs to have more than 256 open file descriptors simply can not be used.

Almost any software that acts as a server is going to require this; a file descriptor is also required for sockets. 

See: https://unix.stackexchange.com/questions/36841/why-is-number-of-open-files-limited-in-linux for more info on why the default is so low. 

OSX went even further using 256 as the default. While I understand the OS is a desktop OS and perhaps they were thinking this somehow would protect users from themselves, it's a little ridiculous IMHO.