If you neighbour has an open network (i.e. no encrpytion) then if you connect to it, you are possibly leaving your computer open to attack. It's a small risk, but best to be aware.
If he encrypts it, then you'll need a password which it sounds like shouldn't be a problem.
You may well find that the signal is so weak it becomes slower than your dialup connection, albeit cheaper
Why don't you just install ADSL at your house, or is that not possible?