1) The TomTom 510 is not an old device, it is brand new and just several months old.
2) The TomTom 510 is not considered a handheld device, it is an automotive device.
It would help if I knew what you want to do with it... A handheld device is just that; soemthing you carry along with you, they are usually a little more rugged, but most of them lack features seen in an automotive device like street level maps and a touch screen.
Are you looking for something for in the car, or something to drag along on a hiking trip?