Its easy peasy old card out new card in, I think the 8800gt needs 2 power leads, i gave a 8800gts which needs 2. They are little square connectors so you can't plug the wrong ones in.
Your 400watt PSU could be a problem, depending if its a good quality. What other USB devices do you have because they all use power from your PSU. You dont want to overload your PSU as id could go pop!! Its hard to say but you don't really wanna take the risk. There is an alarm on GFX cards which sounds if there is a lack of power, its a high pitch squeling.
Another thing whats the rest of your computer spec?