Both of my kids (now 6 and 7) went from crib to big boy/girl bed with rails at age 3. That was the age my doctor recommended (she said if they aren't climbing out on their own, leave them in the crib until 3, but if they are climbing out on their own, for safety reasons you need to move them sooner) It was nice because we totally redid their rooms and got new big boy/girl beds and decorations for their third birthday. And the transition was very smooth, neither one had a single problem.
Thanks to everyone for their advice. I guess we are just around the right time everyone else has made the transition. You'd think that EVERYTHING comes back to you with parenting, but its been such a long while between kids that I had forgotten some stuff. 