ins and outs

I need to buy a firewire interface for a new Mac Pro and Logic 9.

I can see many interfaces have 8 analog ins and 8 analog outs. How can I use the "ADAT" ins / outs through my mixer to give me at least 12 ins and 8 outs?

Plus suggestions for best interfaces in the sub £400 region? If I have to buy two, around £650 - and will firewire handle two?

Happy New Year to all,

Mike Levon