Doom won;t be as demanding as people think imo, anyone thats played the alpha thinks its gonna take a monstor of a system to run it, but that alpha wasn't optomised for anything, was basicaly there just to run at a decent framerate to show the game off. When its out id say people with geforce 4 cards around ti4400-ti4600 will be able to ttun the game decently.
http://www.doom3portal.com/info/systemreq.php
http://www.anandtech.com/video/showdoc.html?i=1821&p=21