There is of course a difference between personalizing and a mess… the picture on the left side is far beyond personalizing.
The hardware of an engineer should be at least what is required for a customer. In most cases it should be even more, especially when you are working in a multi-tier environment. You probably have an application server, database server, web server running at your local machine. This set up requires hardware that is more then average. For example, a solid state drive could boost your improvement.
Two monitors could also increase your happiness. You could use one monitor to display the application and the other monitor to display the IDE or test automation tool. There are several reports on the internet to prove your manager that multiple monitors improves efficiency. In most cases, when you are able to work more efficient, you also have more fun in your work.
Organize a soccer table, dartboard, ping pong table, Wii, at least something to relax during work time. Building, testing, describing software is an intense process. You have to be concentrated and focus on your work. There should be an option to relax during the day. A manager that believes you are concentrated the whole day, fools himself.
Make sure you set up a separate game room or at least don’t disturb people who are working when you’re relaxing.