I need some audio software that, while still having low CPU usage, will allow me to stream two-way sound with as low latency as possible, between two Windows machines (Windows Server 2003). I've already tested Mumble and Teamspeak, and, while they work well on XP, they have very long CPU spikes on Windows Server 2003 (both sides have single-core CPUs).
There is a list of popular Voip softaware on this Wkipedia link Comparison of Voip software that may give you some options to try. I have not seen any benchmarking on CPU usage for such software, so some trials on your end may be required to suit your particular needs.;)