I run:
TF2 server with 23 bots or CS:S with 23 bots or GS:GO with 23 bots depending on what I feel like shooting
+ PS3 media server (1080p media transcoding on the fly)
+ Mediatomb (for MP3 and image serving to PS3)
+ 2 XP virtual machines
+ 1 Debian virtual machine
+ LAMP (Apache, MySQL, PHP) development
+ Samba shares
+ On the fly encoding / decoding of my work-sensitive data
+ Rsync based backups to one of my other servers in another location
+ Any number of other odd-jobs I think of
All on an ancient Phenom 9650 quad core with 8GB RAM (recent upgrade from 4) and ~ 4TB of storage, ancient Asus mobo, Antec Neopower PSU and an Intel Prio1000gt nic. O/S is Debian. During the day I will have up to 6 local + 3 remote desktop users on it, in the evening it depends what I'm doing - the TF2 server gets fairly busy, but it's bots that kill that, not concurrent real people
GMod is hard as people sometimes push the Source engine way beyond what it is efficient on - depends on your map.
Ventrillo / teamspeak use almost zero CPU until you get to silly numbers - usually you are more limited by upload speed on a home internet connection.