Mumble’s server application, Murmur, was already included in the repositories for Ubuntu 10.10, and it basically installed ready-to-go. What’s more, the client application worked flawlessly on my Debian Squeeze netbook.
Memory use was even reasonable. An idle server consumed 3-4MB, with each additional user (up to my sample size of 2) using an additional 2-3MB. I think that’s reasonable, anyway. Per-user memory overhead will probably drop with more users, too.