If you use a high resolution during gaming, the game will play alot slower, depending on your video card.

Anything like a 6800, or higher won't really be affected by this. But older cards, you should use a lower resolution. (no higher then 1024x768)

Monitors can go all the way up 2048x1200. Most new grpahics cards can easily acheive making that resolution, but many monitors don't support such a high resolution.

Here's a good source.

http://www.thescreamonline.com/technology/...onitor_res.html