Which digium hardware should I use?

We’re about to do an asterisk install and I’ve been up on the different cards that are available and I’m confused about how exactly they are similar or different. as in, which of of these should I get: TE411P,TE410P,TE406P,TE405P,TE210P,TE205P,TE110P.

  1. I understand the PCI volt issue
  2. On a practical level, is it worth getting one with echo cancelation or not?
  3. What is the different between single/dual/quad span cards? Can a quad span support up to 4 T1s or something?
  4. anything else I should know? im looking at digium products because I assume they are going to be easiest to use and the most reliable, anyone disagree?

I also should mention we have a single T1 for voice-- with an Intertel phone system that costs us $200/hr every time we want to change a #$% extension.