OSX can read and write to windows ntfs partitions with
ntfs-3g installed. You should install that no matter what you decide to do. Go ahead, I'll wait.
On Vista, to read and write to mac hfs partitions, you need
macdrive. It works well.
My external drives are almost all partitioned in hfs, for mac. I have macdrive installed on all windows machines I use. One hdd I have is partitioned in ntfs, so I can use it with my xbox.