Like the folks above have said, the fence height you need will depend on your dog's personality and early experiences - some dogs just never learn that they can jump a fence, whether 3 foot or 6 foot or whatever. Some dogs take to it so naturally and easily that it's a thing of beauty to watch them go over, but a DISASTER and catastrophe in the making. Strength of prey drive, of course, is a big factor in motivation, as is wanderlust, both of which can be decreased or eased by lots of exercise and interesting activity (agility, lure coursing, dog park visits, etc.).
If I were you, I would either do the six foot fence, or a shorter fence combined with an electric wire. Electricity, although a really scary lesson for the dogs the first 2 times they encounter it (it took all my dogs exactly 2 contacts with the wire and then they NEVER came close to it again), is very effective, but without a visible barrier to associate it with, I think it's much more likely that a high prey drive and/or high pain threshold dog will just blow through it without thinking - and get hit by a car or be afraid to come back *in* again.
Most important, though, like the posters above said - never put your puppy in a position that he learns he CAN jump the fence. It's the same with so many behaviors we'd like to control - for example, never let your puppy learn he doesn't have to come when you call him. I, of course, speak from experience - i.e., do as I say, not as I do! ;o)
Best of luck to you,
L-A
