RAW is more forgiving because it gives you roughly +/-2 stops of exposure correction with minimal loss in image quality as well as easy white balance correction. If you shoot a lot of pics, RAW post production process is generally faster too with the right software... However in terms of image quality, if you got your techniques down, RAW vs JPEG is a moot point.
UltraII is plenty fast in a Rebel XT, as a beginner amature shooter you will never need anything faster if all you are doing are shooting family portraits or landscapes. Unless you want to be your local BMW chapter event photographer, don't bother paying for more.