So you are telling the game to run in a resolution with more vertical lines than your monitor physically has.
One of two things is going to happen.
1) If the monitor is set to automatically resize, then the 1024 will get reduced to 900 lines and the horizontal image will get compressed a bit.
2) If the monitor does not auto-resize, then the video image is going to be clipped. You are going to lose 1024-900=124 lines off the bottom of the game image.
So why are you running a foreign resolution and not the native one?