Backpacks and water bottles

Okay, so I think while I am in St. Louis this weekend, Sarah and I should go and try on some packs at REI or where ever. I see great packs at great prices on sierra trading post, but I just don't want to buy blindly. Like this one:
Lowe Alpine Lightweight Womens Pack
Or this one:
Lowe Alpine 4000cc $64 Pack
I need to know my torso size...

Also, how is the water in Nica? Should we invest in...
A water purifying bottle? or is that a gimmick?
Have you heard from our friend Eric in Nica lately Sarah?