I host a counter-strike server for about 15 players on a i5-760 without any lag.
Counter-strike servers don't really have much system requirements, but it actually depends how much processing power a game draws, the number of clients currently on, as well as the upload speed of the connection.
If you want a budget build, i'd suggest the i5-2400 or i5-2500k if you fancy overclocking in the future.
If you like AMD, i certainly suggest the hexacore range as they are pretty good (1055T,1075T,1090T or 1100T) for about the same price as the intel. My friend uses Phenom x4 955 and it was fine too . You can host multiple servers definitely, max i have tried is 45 slots without lag(also my internet connection is a little shitty)
In my opinion a decent quad core will be enough. i7 wont really be necessary.