if its an older board with an older graphics card, then its most probably AGP.
but you should be able to find out what bus you have by doing a right-click on the desktop and look under the advanced display setting for the info about your graphics card.
For half life 2, dont even look at nvidia cards, defenitely the X800XL for the 200 dollar price bracket, great card that performs as good as the 6800ultra in some other games.