As The Serpent Rider says, boils down to what your motherboard supports.
Experiences from my own Win9x/WinXP build with testing both the HD4650 AGP and HD3850 AGP with a varetity of games is that you'll come to about 2009-2011 in games no matter what. In about that age range games started to become multithreaded and a single core CPU just won't cut it. Also this is about the same time these cards were introduced, but remember they were not highend cards at the time. So running games in 1080p with everything turned to max at 60fps+ is just not going to happen, as you'll be GPU bound (Bulletstorm from 2011 is an example). So if you have a quick P4 that works fine with that card if you limit yourself to 2007-2008 and below (Crysis not included).