The timeline is good to have because it ensures that everyone is on the same page and reduces the likelihood of a misunderstanding. This will help you tremendously as you go through the project. Try to think of everything you'll have to do and include it. Be sure that training is very thorough. This is probably one of the most common reasons that a new system fails in a law firm.
Firms often make the mistake of underestimating the importance of training.
Budget a significant amount of time and money towards training and the results will be well worth it. Many new systems require a Windows NT SQL server, so for those of you running Novell, be sure to give yourself plenty of time to get the NT server up and running. Try and have it running for a least a month prior to the installation of the new software. This adds another dimension to the project. Look at a time entry system.