Depends on where you plan to work, but I wouldn't recommend it without at least one year of experience, longer if you want to do any Critical Care areas. You need to be able to hit the floor running when you travel (no one is going to hold your hand), so you need to feel pretty confident in your skills and knowledge base.

check out travel forum
You need at least 2 years expererience. You need to be very tough skinned because many hospitals are not very travel friendly. Especially when "their bonuses" are being taken away because travelers coming in.
I researched MANY MANY travel companies and I am very happy with mine. You need at least 2 years clinical experience.