When playing games, put the resolution at 1024x768 and every single setting on the lowest possible.
Then download Fraps (Click this text)
Then open up fraps and just go back to that game, in the top left corner should be a big yellow number. If it's over 30 you're fine.
If the frame rate (Yellow number) is high, let's say 60-90 start slowly putting the settings up to higher numbers (ALWAYS keep anti-aliasing OFF and anistropic filtering to low or off) and just keep raising the settings, until the big yellow number is lower then 35, or until the game is going to slow for you.