Why won't my mac start if USB devices are plugged in?

This behavior just started in a system that had been stable for months -- except for OS upgrades. Not sure if this is about the motherboard USB ports or possible the power supply failing. Here is the situation: The Mac Pro boots normally from the power on button if the only usb device attached is the keyboard / mouse. If I plug additional USB 2.0 devices (Wacom tablet, IPod) into the usb 2.0 ports on the front or back of the computer, then when I hit the power on button I just get a "click" noise and no boot. If I plug the Wacom and the iPod into powered usb ports -- like the ports on the back of the monitor -- then it will boot normally. This tells me that the USB ports on the motherboard are not getting / delivering the voltage they need at boot. Does this sound right? Any thoughts or ideas? Is there any way to test USB ports? Or to test the power supply?

My mac is still under applecare. Is it worth taking in to Apple Store?
What would they even find? Could I need a new motherboard or a new power supply? This is a new problem, to me it suggests some kind of system partial failure. But maybe this is just what happens on older hardware as the software gets updated. Another idea for a solution would be to get a Sonnet USB 2.0 Pci-e card and plug that into the motherboard and use those USB ports, but I have no guarantee that would work better -- and solve the fails-to-boot problem. $50 to get a card and try it...

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