I would skip the drivers on the CD, and instead only install the latest from the Nvidia website. You will need to install the new drivers right after you boot up for the first time. Your procedure looks about right.
1) install newest drivers for the gts 250 on the site
2) uninstall the gts 250 and install the gtx 480
3) turn it on and itll be ready to go?
I would not worry about updating the drivers for your old card. I would also not assume it will plug and play with the old drivers, even if they are compatible. The PC will still need to install drivers for your new hardware. So, take out the old card, install the new card (don't forget the two 6 pin power connectors), turn on your PC, install the latest drivers from Nvidia.com.
After that, go to the Nvidia Control Panel (either use the tray icon or right click on the desktop) and adjust settings to suit your needs. One setting I always need to adjust under Global 3D settings is: Enable Single Display Performance Mode. For some reason, it usually defaults to Multiple Display Performance Mode. Also, set VSync to "Force Off". Under PhysX, leave it at Auto Select. There are a few others, but in general most of the Global Settings can be left at default.
'Next generation four +12V rails(12V1, 12V2, 12V3, 12V4) supports high-end graphic card and PC system (combined loading of 48A)'
So is that enough amps for it?
Should be OK. What brand/model psu are we talking about?
Some cheaper psu's simply sum up the individual rail amperages to give a total. That is rarely correct, and the real total will be something less.
Still, the 42a requirement is probably conservative, and if you have already bought the psu, it should not be a problem.
If you have not already purchased it, there may be some better quality options.