But, did you uninstall the onboard drivers, and I would check in your BIOS to see if there is an option to turn off the onboard while you are using the add-on card.
Also, what brand of PSU did you get, some 450W are notorious of only being able to push out about 325W on a steady basis. And if you have a decent CPU, 450W might not be enough anyways, I try to stick above 600W if possible, you can get a good 600-650 Watt for the same price you probably paid for your 450W.
Also, did you use the AMD/ATI site for the driver update, to be sure uninstall it, and use these
drivers to be sure if it persists, you can try the beta drivers, they might help.
To further the device manager, there is a checkmark to show hidden devices, click it and you can try disabling the onboard video adapter from there as well.