the main problem is that i would prefer a 500 gb partition (Win 7), 100 gb partition (Win XP) and 200 gb (Archive). is it even possible to make a 500 gb partition with ntfs??
Sure, they make hard drives in the terabytes these days (1024GB = 1TB).
So yeah, you could make a 500GB partition, assuming you had a hard disk that is larger than that (to have room for the other 100 and 200GB partitions). If I were you, I would get a 1TB hard drive, then chop it up into whatever partitions you need.
But keep the 320GB you have now. You can use it as a slave drive, or as an in-house backup (so to speak). Then you wouldn't need an external hdd to back up your files (backing up to the same drive with separate partitions is not wise, because if the hard drive crashed, your files would be gone. Use a completely separate hard drive. An old internal like you have is perfect.).
I hope that answered your question...