If you are already running the motherboard then you are probably running on the onboard graphics.. For using your 9800GT, you need to first uninstall your current graphic drivers (you can do it from the control panel or use a software like DriverSweeper to do it).. Then shutdown your computer and put in the 9800GT card in the slot and connect the necessary power plugs.. Restart the computer and go to BIOS.. Then disable the onboard graphics chip from BIOS and activate initial display to PEG mode.. Save and Exit from the BIOS and boot to windows.. Install your 9800GT drivers and you are all set..
I would only turn off the onboard video and/or uninstall its drivers if it ends up giving you problems. It shouldn't be necessary with most boards.
So to answer the question you put it in the slot and install the drivers.