The best thing about using an invoice system is that you can track your time against the amount you're getting paid for each task. In order to run a successful business, you want your hourly rate to be profitable so you're not losing time and money. Another benefit of using an invoicing system is that you'll never save over an invoice and lose the work you've calculated for another client.
Generally, your contact info should be on the opposite side of the recipient's info. So if your business name and logo are on the right side of the invoice at the top, the client's name and info should be below this on the left side.