I have used Mumble and Teamspeak in the Ubuntu Server. Mumble is suppose very low latency but the client is horrible. At least compared to Teamspeak. the memory usage was good. The configuration and installation was a little complicated.
For teamspeak the low latency is not as better as mumble but the client is very good.
What other Low Latency VOIP server for Ubuntu exist and how is the look and feel for their client and the quality for their server.