Simple is Best

A new telphone system based on voip two weeks ago, the TARGET is a low-cose call center on voip system link with pstn like 800 or 400 hotline.
I need to choose the voip server, client, pstn gateway and ata etc, it's boring but very challengingly, i like it:)

voip protocol:
h323(old man?), sip(common use but nat problem?), iax2(asterisk support, other supported server?),mgcp etc

voip server software:
asterisk(support sip,iax2,conditional h323), gnukeeper(support h323),
voip protocol, ser, sipx, trixbox(based on asterisk), callweaver etc, many servers but we need only one,

voip client:
softphone iax2 client(zoiper?), sip client(x-lite xten?), but the h323 client?, cisco hardphone?

voip pstn linker:
voip gateway, Digium pci card

voip hareware:
one Dell poweredge 1425 enough? or another application pc server?

then the other problem, performance and the stability and the expandability and the last one: COST

total is a long way, but i am on the way:)