asterisk users - May 2005 - Inbound call center - reliability \ scalability with queues

We are wanting to move off of our legacy inter-tel phoneswitch and move to
VoIP and asterisk.  We are looking for a new PBX because the inter-tel
switch is too difficult to integrate our existing (and new) software into.  
 
We are a technical support center.  All our calls currently come in on toll
free numbers via T1's, and there are 3 of them.  I want to use a media
gateway to convert the T1's into SIP VOIP (I want reliable hardware for the
gateway), and use asterisk as the PBX having all incoming and outgoing
channels as SIP.  Almost all dialplans will be using Queues, and there will
likely be no more than 10 queues, with (currently) about 80 incoming
toll-free numbers.  There are approximatley 30 agents, but as of right now
there are no more than 15 agents logged in at a time.  We need to be able to
support 60-70 simultaneous calls initially and we have to be able to do this
reliably.  We also need to be able to scale into the 100's of simultanous
calls range.
 
What would be the best option, to have 2 powerful machines (dual
powersupply, ) with one as a hotswappable backup or have multiple machines
with a sort of load balancer setup?  Having multiple machines could possibly
cost less, but I'm not sure how the queues and agents would be managed
across multiple machines.  I.e. how would the agent 'login' to each
asterisk
machine so that the calls could be handed to it, and how would the calls get
handed to an available agent by 4 seperate asterisk machines?  I've read
through the wiki, but I'm not sure how much overhead queues would put into
the system.  I want to have all the codecs the same, so the asterisk
machines doesn't do any transcoding, and have all channels as SIP.  There
will be music on hold.  Would a dual 2.8 ghz xeon in this config be able to
handle 80 simultaneous incoming calls?  Would using the 711 codec make a
difference in available processing power?
 
I'm sorry if this has been answered a million times already, I just
didn't
see many configs close to what we're trying to do to compare to.  Thanks for
any input you may have.
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