Hi there,
There isn't, AFAIK, a best practice document with hardware suggestions. From my personal experience I would go for a machine with, at least, 2GB of RAM, a dual core processor, and plenty of disk space, as logs can grow really big depending on how you configure your system.
Also if you think that the machine will probably be your SEP11 server when you migrate the spec will do the job too.
There is a KB on optimization for the reporting server that you may find useful: 2007073113212648
I would recommend you a secondary server per site, as it will make your definition distribution much quicker. But if you have only one site (all connected via LAN) a machine with the specs above will do the job (although I would install one secondary for backup purposes... but you can do that in a workstation)
Hope this helps
paulorf