Well since you already have XP, might as well go in order of their release. Vista then Win7.
You will need at leas 50Gb if not more for both Vista and Win7. The partitions are up to you. I dont know your setup so i cant advise on what you should do. I dont know if you have separate partitions for pagefiles, data, downloads, documents, pictures and so on. You have to make the call.