For shooting soccer outside and if the weather is overcast or raining like it looks here I would suggest establishing a good exposure, then set that setting in manual for the rest of the game.
Shoot AI Servo, with continuous shooting you will be surprised at how much movement there really is between shots and how often you still miss the peak action, so I would go with single shot and work on your timing.
If the light is changing, then instead of manual, go with Av mode to have control on your depth of field keeping the ISO high enough to keep your shutter speeds up. I'm not familiar enough with the Xti, but if you can switch to center focus point only... it's much faster than other modes for focusing.