asterisk users - Mar 2004 - IVR plus small customer support center (was IVR setup)

I'm trying to deploy an IVR with 12 analog lines (using VoiceTronix
OpenSwitch 12 - http://www.voicetronix.com/hda.htm) and 5 SIP phones to
handle these calls - if IVR prompts weren't enough to help the customers.
I'm still researching, nothing decided so far.

Which should be the 'best' hardware to this handle this configuration? A
Intel Celeron? A Pentium 4? More memory?

A T1/E1 trunk is not a solution, as this box will be client from a large
Ericsson PBX, handling only the calls to the customer care center.