Delphi3D has a nice hyperlinked list of OpenGL extensions that reports which card/driver combinations support a given OpenGL extension and has a link to the SGI specifications: http://www.delphi3d.net/hardware/allexts.php
[http://techreport.com/ Tech Report.com] have a good, sortable table of Graphics Card specifications: [http://techreport.com/etc/comparo/graphics/ TR's graphics comparison table]. Although it only includes up to the ATI X850 and NVidia 7800GT, that should be enough for most people
===eBay guides to Graphics Cards===