Galera and in-memory
A Galera node's memory usage is very similar to a normal MySQL/MariaDB standalone server. If 128GB is enough for a standalone server to handle your workload, it will be ok for a Galera node too.

Of course you can also have server with different characteristics. In which case, you will also need a load balancer that will take into account the servers "weight". You can use MaxScale, for example.