Why do you want to use such a high shutter speed - the duration of the flash will be fast enough to stop most movements. I'm not sure what the duration of the on-board flash is (you may be able to google it) but it will get faster as the power is reduced. With my Canon 580Ex the slowest flash duration is 1/1000 & it goes up to 1/35000 at 1/128 power - see this link
Canon-580ex-flash-duration