Thanks, cl-scott. Very kind of you to help out.
First off, you will not find a thunderbolt cable at monoprice. This is because the thunderbolt cables have chips in them on either end. That is why thunderbolt cables are rather expensive, but it also means that your existing thunderbolt port can use a future optical thunderbolt cable, as the modulation happens in the cable itself. Also do not confuse a thunderbolt cable with a displayport cable. They are not the same, and will not work with the iMac in target display mode for sure.
Anyhow, have you done this yet? I am thinking the exact same thing you see, building a gaming rig and using my TB iMac as a display, but I cannot for the life of me find anyone who have tried this and can answer if it works or not.