What is the recommended hardware?

The hardware needed to run OpenERP varies widely depending upon the modules installed and the load (number of users, number of entries per day...) so only general guidelines can be given.

The database perfs depend directly upon the speed of the disks, so get the fastest disks you can. Good SSDs make wonders in this field.

OpenERP not a memory-hungry beast, but depending on your OS and tuning, more RAM can dramatically improve I/O speed.

The server part in is mostly single-threaded, so focus your CPU choice on the maximum core speed, not the number of cores, and get a processor capable of boosting a single core's speed when the context requires it.

When scaling up, consider using one or more dedicated servers for each tier:
- PostgreSQL database cluster nodes
- OpenERP Servers (preferably 1 server per database)
- Web clients (load balancing can be achieved more easily starting with v6.1)