If you book a package, you will need to pay the balance off at least 45 days prior to departure, regardless of the TA you use. Those are the WDWTC's requirements.
If you're booking room-only ressies, you only need to make a one-night's deposit and the balance needs to be paid at check in.
Most TA's will take "installment" payments that can be done by credit card, debit card or check.
My own feelings are that you should take those monthly payments, put them in an interest-bearing savings account. Then pay the trip off when you have to. Why give Disney an interest-free loan for up to 1 year?
Plus, since there are no discount rates out yet for Dec. 2005, there is the possibility that your rate could go down, leaving you with either a room credit or waiting for a refund.