That's the basic gist of it... but you don't need to do a series of reboots, you can handle it all in one step. Download the new card's drivers now.
Uninstalling the drivers for your onboard isn't necessary, just shut down, unplug, ground yourself, and install the video card. Boot up, go directly to the bios and disable onboard video. Save your changes, exit and load Windows.
When you get to the desktop, install your new drivers. You're done.
ps... on a side note, it's really not important what wattage your PSU has, because as far as video cards are concerned, that's a meaningless indicator. You can buy a high watt PSU that doesn't have sufficient +12 volt amperage just as you can have a relatively low wattage PSU that has high amperage on the +12 volt rail. Video cards do not care about watts; only amperage, the more the better. In the future, next time you decide to upgrade your card, check out your PSU and make sure it has the amps the card wants/needs.