Upon further reflection of the amount of voltage feeding back into the case, I think I might have figured out the problem. The 6600GT not only uses the standard AGP power interface, but it also has a separate power requirement from the PSU. When I used a volt meter to see if current is running through it, I found that roughly 16V were coursing through the back case of my computer.
Now, when I replaced the 6600GT with my old 5200, I tested it again. It registered .4V - .5V. Of course when I touched it, there was enough electricity flowing through it to make me curse, but I realize now that there is no separate power requirement for the 5200, you only need to plug it into the AGP slot.
If the power was flowing into the PSU from the video card and mobo, then it may have been feeding back into the video card via the AGP slot and the power cable attached to the 6600GT. Since the power output was considerably higher for the 6600GT, is it logical that the PSU might be at fault?
I'm completely thinking aloud, and I apologize and congratulate all of you who have completely read through all of this. My question still remains, however: what seems most likely to be causing the electrical feedback?