ATI Tray Tools has a "Flash On Screen Display" option, so you can push a button and it will display anything from core clocks/mem clocks, temp, directx version being used, etc etc... You can choose what you want it to display.
Not sure that it's compatible with the HD 3xxx cards though. I know it doesn't detect clocks correctly on my 3870, but I haven't tried using that feature with my new card.