I'm assuming the inside of your PC looks something like this:
If so what you want to do first is remove the zip ties off any set of cables tied to cables coming from the power supply. Based on the picture, it looks like the power supply is installed simply; does not look like any of the wires are being routed through the back of the case or anything. First just focus on installing the new power supply; once you do, turn it on and just make sure everything starts up fine.
The video card is pretty straight forward; If i remember right, think you mentioned you upgraded the video card once before so pretty much the same thing again. The one thing you want to keep in mind though is to remove the old drivers before installing the new one.
P.S. The above information is based on the info you provided on the first page, if the inside of your PC looks different or the power supply cables are routed differently and your at a loss as to how to remove it, would help if you can post some pics of the inside.