I'd say about 5 stable servers.
A good howto is located @ http://www.cheaplinux.net/cs_howto/cs_howto-3.html You'll need to find some things and the new HLDS_L is in .bin form instead of .tar.gz,so you just need to run it instead.
No CD Key needed,you just need the small,free Half-Life Dedicated Server which is downloadable from many mirrors around the world.
Also to administrate the server they need some rcon software which is easy to get,all you need to do is add the rcon password into the server.cfg.
Depending on a map and CPU/Memory load, I suggest you do two CS server per server. 20...22 max players per server (with your given HW configuration). CPU should be able to handle it GOOD, also, I suggest 1GB RAM in this case..
This is of course with Linux server - you will save more in a long run (unless you really want to pay for Microsoft licence & stuff).
You do not need CDKey, all you need is to download HalfLife server + CS files (a bit over 110MB or so I guess).
is any software that I can use for the client to restart their server?
You mean to restart a game server or a complete server reboot?