H-1B Transfer, When? How?

If I have the recipt, can I begin to work right the way? Also the new job has to be the same/similar with the old one? How do you prove if it's similar? Like accounting and finance, can you tell they are similar? Who will be the one judges?

Also if I want to begin the new job on 8/1/08, and the new company applys for transfer on 7/10/08, can I still work for the old company till 7/30/08? If I quit today, my H-1B expires right the way, right? So when is the latest time I can quit my current job?

Sorry... really confused... have to make decision soon... Thank you guys soooo much!