I have a sme server running on AMD 200's with 128MB of RAM supporting 7 users. The users never complain about speed.
I often "sell" sme on servers with AMD 800's to 1.5GHZ and IDE drives to small businesses with less than a dozen or so customers, with no performance problems.
I have a few 100+ user locations running 3K to 4K$ dell servers, with no performance issues.
I hope this helps in some way.
Last item, I have a customer (7 workstations) running SME on one of those 199.95 wall-mart.com boxes. It handles file, login, dhcp, and mail services. No complaints about performance.
As a matter of fact, my customers who have linux servers, seem to only have windows desktop problems as a rule, how odd!!!!!!!!!!
Chris Curtis