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For very large sites, it's worth noting that a single huge machine may not be necessary: using multiple machines running spamd, with your MX on a separate machine, using a DNS round-robin record to load-balance, will probably work quite well.
This is all very well, but an answer could be a bit more clear than this 
- What sort of numbers are involved with each parameter?
- What kind of hardware will typically handle a specific set of numbers?
- I work for a company of about 15 people, one linux machines serves their e-mail, can this be done on a plain PIII 500 MHz with 1/2 GB and lots of other services on that same machine?
- furthermore, how do I set it up using postfix (or another mail server)?