Following along the lines of what u_rebel_scum said in Dmod's thread: game and settings. Generally, the later the game, the hungrier it is. Any effects will also eat performance. Start with the latest game you like to play, no effects*. If it won't run right on the version you're using, you know you need to go earlier. As mentioned, some games may not work well/at all, or even be supported in earlier versions. You can do a lot of research to try to fit your machine to what you want it to do, or you can get a more powerful one.
*Looking back at Aaron's testing, I might assume he's using switchres (?), which I've found runs fine, I think regardless of the resolution, but it doesn't fill the screen. (Using D3D versions after the re-write makes this different.) It wouldn't be a real test for most, as they're running DD+hwstretch.