Question about hardware, conference project

I am planning to set up at telephone service that works like a

What I want to do is to put together a computer system that is
hooked up to a telephone line and anyone calling that number will
automatically be connected to the conference, very much like a public telefone chat.

I am also planning on setting up additional features such as
allowing participants to jump between conferences etc.

I have been recommened to use a digium board, at this stage I am
a newbie trying to get a handle on what type of hardware
I will need and if the parts will be compatible with each other

Hope anyone out there can help me with the following things I am wondering about

I currently have a ASUS CUV4X Rev. 1.03 motherboard with a
Pentium 3 800MHz processor, does this restrict my choice of digium
boards? I am particularly concerned with the 3.3V / 5.0V PCI slots requirement

I currently have no hard disk for that motherboard. Before I
purchase one, does any of digiums boards have any requirements
about hard disk capacities?

I noticed a term called SPAN, as in Quad-span, dual-span or
single-span. Can anyone give me a brief explanation of this? As i
see it, this denotes the number of cable connectors on the boards.
What kind of system would require quad-span, dual-span or
single-span boards respectively? What is the advantage of one over
the other?

If this is not the place to ask these questions I will be happy if you could direct me to a proper discussion forum on the net.

Thanks Petros.