The subject is simply far more complex than you are stating. Many factors contribute to the server specifications, CPU bandwidth is just one of many considerations.
For quite a long time we used 700Mhz Pentium III’s with 512M of RAM. 128M of RAM is really light.
You are correct in stating the g.729 adds transcoding overhead.
Disk performance is also critical. One of our new install with 176 extensions, 3 call Queues, iSymphony and Queuemetrics is being installed on a dual quad core Xeon with 8M RAM. More importantly than raw power it also has two SSD’s RAID 1 for the OS and Asterisk and two 500M SATA’s for call recording storage.
At the other end of the spectrum we have great luck with the Intel Atom processors and SSD’s for a no moving parts system.
The VIA processors don’t work real well with CentOS and Asterisk I would stay clear.