You're using Vista right? Honestly, I don't think there is a widget for exactly what you're looking for. You'd have to use something to monitor CPU/RAM, then something else to monitor your video card.
Something like this:
Windows Live Gallery
Then either ATiTool, Rivatuner, Everest, or something else for the GFX temp. You can just run that, and it'll show the temp in the notification area.