You could do a requery command to update the part number automatically once you have selected the catergory and the same with the state based on the city chosen.
Also a bit of colour would make it more appealing.
Im not too sure whether you will be doing it later (good practise!!), but if you use command buttons it will allow users to have a navigation system. It also prohibits them from causing problems (which believe they will!!!). You could create a Switchboard which when the user enters the system they will automatically go to. They can then navigate through the required forms and always return back to the switchboard.