Even if you have a really good computer, turn them down anyways. Dual 8800GT's and I still run standard simply because the less complex the game is, the faster it runs and the less laggy my FRAPS recordings are.
One other tip, if you're playing casual mono, go ahead and turn on ironmode. It's easier. I'm not kidding, taking away the shoulder lanes allows you to make fast moves without overshooting and ending up in a shoulder lane.