Okay, hopefully I don't miss anything, that's quite a list
1. Video cards don't connect to the power supply, they draw the power through the mother board.
2. The AGP 6200 should work just fine, just remember to disable the integrated video in the bios.
3. The switches on the power supplies are just a master on/off, the power button on the front of the computer is
connected to your mother board. Don't worry about that one.
4. Why not pick up a SATA II controller? Treat yourself
5. No, it's not overclocking, I actually wouldn't recommend overclocking at all. Whether or not your motherboard will actually work with the DDR 3200 that should be in your motherboard manual, or if you want me to look that up for you post your make and model of motherboard. A good rule of thumb is to only get the type of RAM that is already installed. I'm referring to the speed and type, not the size.
I hope this helps, if you have any other questions don't hesitate to ask.