Because you are expanding your existing RAID (if I read that correctly) you should use drives that are the same size. You can use drives that are bigger, however the RAID will ignore the extra space and will effectively turn the drive into a 250GB drive.
Now if you are looking to replace drives later with a larger drive for all of them, you can replace them one by one until all the drive have been replaced with larger drives and then expand the RAID to make use of the space.
You should be contacting your vendor for the drives you are going to buy.I can't remember if you need sleds for the XServe or not. Most of the time you can get drives from just about anywhere. Just make sure you aren't buying common consumer grade drives.