I initially ran my server on a 1 meg down and 256kb up broadband connection with a P4 3.2Ghz, 1Gb DDR400, XP pro and set player limit to 20. up to 16 players things ran well unless there were more than 4 players fighting in the same system at the same time. If the server was full it would lag quite a lot.
I now have my server hosted elsewhere, still using the same hardware but with a potential 3 meg down and 3 meg up. I can now support over 100 players but i have set the limit to 64 due to bandwidth restrictions and costs. When I get above 20 players my cpu is maxed out and the server load is in the red. I am now looking at getting a dual xeon 3Ghz setup with 2GB DDR2 and Windows Server 2003.
It all depends on how many players you want and how much money you are willing to throw into this.
Hope this helps.
Buck Rogers
Server Admin
Colours Elite UK
www.colourselite.co.uk