Yes CS and CS:S are using only one core, thats way I suggested 3Ghz(if you can get 3.2Ghz it will be even better) the second core will be for other OS background tasks.
This way your PC will be more responsive.
About the load on the server computer, the bots will eat more CPU only on the server machine, for the real clients - bots and other players are no difference.
Note: Core 2 Duo CPU is much better than Dual Core one. The Core 2 Duo is based on the new Intel technologies and have much more cache. The Dual Core is almost a regular Pentium 4 but with 2 cores.