I manage several trixbox servers some on 2GHz P4 and some on xeons. They dont really need the xeons. I have some with 5 phones and some with up to 50 phones. I dont think the Xeons were needed but I wanted Raid 1 and those were the only 1U servers we had with scsi raid built on board. P4s seem to work great for asterisk servers....
I dont think on any of the boxes I have actually used the maximum concurrent calls. I believe the most current calls at one time has been around 10 - 15 at most.
Reading above I see some say they have had 100 concurrent calls an a 2.8 P4. I know teliax has built its business on asterisk and so have many other VOIP companies!
I have looked at getting the G.729 codec but have never needed.