asterisk users - Jan 2010 - Meetme conferencing - large deployment SIP or ZAP?

I've been asked by my company to setup a conferencing system to support up
to 400 people on a conference calls, where all users will be dialling in
frpm the PSTN. I am exploring using Asterisk meetme to do this. I have two
questions in relation to this:-

For Meetme conferences is it better to have all participants to dial in via
SIP provider terminating to Asterisk via SIP/IAX, or use something like the
Digium TE420P cards connected to 16xE1's (I'm in Australia). Are there
pro's
and con's to each method.

For this number of users, should I be looking at multiple asterisk servers
to distribute the load, or a single server (I have some Dual core Dell 2950
servers available). If its multiple, can you simply daisy chain conference
participants across servers?

Thanks

Steve
Sydney, Australia
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