In my experience, the most important thing is RAM. I was able to support 16 players on a dedicated server with 64MB of RAM on an NT 4.0 box.
I would reccomend at least a Pentium II class server with 128MB of RAM, however. Our server is a Dual Pentium Pro 200 with 384MB RAM.
As far as how long to leave the server on, it depends on what you are after for usage. If it is not on all of the time, you will not get as many repeat users. If you are just looking for people to join and play with, however, just start it when you feel like it. If it has a good ping, people will play. There are not that many servers our there right now.
Keep in mind, however, that the largest blocking factor to a server is bandwidth. If you want 10 players for example, I reccomend you have at least 300Kbps of upstream bandwidth and 300Kbps of downstream bandwidth.
Hope this helps,
Shan
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