don’'t try it. You will make yourself unhappy. I recommend that you set up a test server with 50 users, then another with 1000 users and get some experience with Wifi.
How do you want to backup the database?
What are the reasons for you to choose this database?
Which operating system are you using?
Can you wait for Project Pampero[/url], maybe this is the better way to handle 20k users.
PS: You had some questions …
I’'d estimate 5 GB memory (1 MB for every concurrent user).
Two 1 GHz processors should be fine, a single processor system may be a little bit too small to run garbage collections on 5 GB memory, but if you can get a 3 GHz processor you may want to use it.
I have no idea about the database size but the content may be less than 2 GB, so the disc space is something which does not matter.