Windows 7 also benefits the active or occasional trader by interfacing with multi core processors. Since you're likely to track numerous events and trades at a time, you need the power that comes with a 64 bit system.
Custom trading computers are often designed and built by traders themselves. The value in these computers also comes from benefiting from their knowledge of testing what works for trading computers. They took the risks to spend money on hardware just to find out in the long run it doesn't stand up to the task of trading.