The hardware depends on the traffic you expect and the size of your website. Basically you can install Sitefinity on a single machine with 1 GB of RAM shared for the the SQL server as with a processor of 1, 5 Ghz..
It is better to have at least 2 GB of ram, 2 Ghz processor and 1 GB of storage. This is the configuration that most of the hosting companies offer.
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