asterisk users - Nov 2004 - Is it feasible to use 1 SIP account for PSTN connection on an Asterisk gateway accepting IAX2 connections?

Greetings,

If you are using an Asterisk server to accept IAX from other Asterisk
servers, that could have many voice channels, as well as individual SIP
client connections, is it feasible to have one SIP account that connects
to the PSTN.  The SIP account can connect multiple times.  Ie. it is not
limited to being used for 1 voice call at a time. But would using SIP
for the backend connection to the PSTN network become a major bottle
neck for the gateway server?

G.711 will be the VOIP codec for the SIP to PSTN network.  On a P4
2.5Ghz or so, with 1GB ram, how many voice channels could be handled? 
The IAX connections to the gateway would be using IAX2.  SIP connections
to the gateway could use any codec, and the SIP usage is secondary. 
Allowing only IAX2 connections is possible, moving the SIP calls to a
different system.  The main thing is the G.711 SIP call for a PSTN
connection.  I assume that for every voice channel in use with in a IAX
connection a new SIP connection will need to be established.  Is that
the case?

Thanks for any input you can provide.

Taylor Carpenter