QuickBook is a real dream system because it manages inventory and invoicing. With it, you can train other members, have multiple users, run sales forecasts and manage reports very easily. It is a small investment at the beginning, but I love it. A good accounting tool is the one tool that you should really invest into.
I use iCal because I like simplicity. Sometimes you end up spending more time managing the different tools than doing things yourself. They complicate life instead of making it easier.