USB overload / nothing plugged in
I'm pretty sure Snow Leopard does not provide native support for USB 3.0.

This is probably the main cause of your issue.  You would need to make sure you have installed the proper drivers for your PCI card in order for the kernel to recognize and allow this new functionality.

Other possibilities:

USB 3.0 is inherently designed to draw way more power than USB 2.0.  It's possible that when you plugged the device in, that's when the machine actually recognized the PCI card itself (or at least realized the necessary power consumption for running such a card and the fact that it was trying to draw more power than it was able to support).

Also:

Are you running any other PCI cards in your slots?  The specific hardware you mentioned has a 300W max for all 3 PCI slots combined, so if you are sucking power from any other slots, this could definitely cause the issue you are seeing.