One thing you can do to get a general idea if a particular game will work on a slower CPU is to run the game on your faster CPU, and hit F11 to show its running speed and F10 to toggle off speed throttling. Then get into the actual gameplay where you have lots of sprites onscreen, etc., and see where your speed is at. For example, if it's running around 200% speed, you'd need a CPU about half that fast to manage 100%, etc. Of course, it's not quite that simple, since there's other factors at play (blitting speed to video, etc.), and a lower-end CPU is typically not just a slower clock speed than a higher end - it also does less per clock cycle. But seeing the unthrottled speed on your faster machine will at least give you some kind of ballpark idea of how much CPU grunt you need.
But as Krick noted, if you get a CPU that's faster than what you need, you won't regret it in the future.