I've run a small (6 players) out of my DSL line (384up/384down), but I play on my friends server which is hosted on an ISP's servers with a T3 connection.
Typcially, servers are 400Mhz+ in speed (numerous are dual cpus), with 256+ megs of RAM, running on NT/Win2k/Linux and a few running on FreeBSD (with linux binaries compiled). Connections need to be a true T1 or higher to get a wide amount of appeal, otherwise the connections are laggy and no one will stick around on it. To give you an idea of how much bandwidth a CS server will consume, a 20 person server (10v10) will completely saturate a T1.
vash