No difference. Not really much of an upgrade to what you already have although you can customise it... search and download:-
Run them and at least get root access to start
Adding market really does help in enabling you to install third party apps - follow the appropriate threads as it can be done
There really is no definitive process to follow - it depends on what you want to achieve.
There are a number of threads here dedicated to alternative roms / firmware. Place the 'image' folder on your SD card. press and hold the side button while powering on will put it into fast-boot mode. If you don't really know what you are doing, I would suggest learning about it first. Make sure you use a rom / firmware for your device or you could end up with an expensive brick.