how cold do you normally set your AC?

hermit crabs are best staying in temperature around 22~27 degree celcius. or else you will need a heating pad.

oh, you're going to get land hermit crabs right? marine hermit crabs temperature depends on the type of marine hermit crabs you get.

have you read up some articles about them?

knowing what you are getting into is good before you start buying them. you will get some rough idea on their habitat and their behaviours.
below are a couple of sites that provides very good information on land hermit crabs.
http://www.crabstreetjournal.com/
http://www.hermit-crabs.com/
http://www.hermitcrabs.com/
well, they will need some dry land some water. putting water saucers would be fine as well, but remember to change them often.
you can refer http://exoticpets.about.com/cs/hermitcrabs/a/feedhc.htm