yes, you can download Direct X from the internet. Most versions are available. Just check your version or there may be DX9 on the drivers CD you get with the video card.
To disable the onboard graphics you go into the BIOS and set it to disable( most newer computers do this automatically when a graphics card is installed), but you need to check the manual for this. If you do not know how to move around and change things in the BIOS i would suggest finding a GEEK friend that can help you.
Once you have disabled the onboard graphics, then save the changes and shut down the computer, ground yourself well( keep touching a bare part of the case often while working on it to get rid of static electricity), open up the computer for access, and install the video card. Put the cover back on and restart the computer. You should get a ugly 640x480 screen, then you install the video drivers from the drivers CD. Read the directions that comes with the video card
because they may want you to install the drivers before you install the card.
Once the drivers are installed you set the display properties to where you want. Usually on a 17 inch flat screen 1024x768 up to 1600x1280 range. Just set it to what you like with the icon and text sizes.
You should be good to go after all that.
You need to download the service and users manual so you can read up on what to do.
or the systems user guide