we are using SuSE Linux Enterprise for Opteron 64bit and it runs smoothly.
AIESEC is running the system with currently 100.000 users, realistically I'd say it is more like 40-50.000.
In the default installation you will have the parameter set to store files in the database instead of the filesystem (in the content-repository-content-files folder of your installation).
You can change this by going to your "site-map" and clicking at the "parameters" link of your "file-storage" packages. There you can set the parameter StoreFilesInDatabaseP to "0".