In the mean time I made some experiments using slow and aged PC:s in order to get a feeling for what might happen of (or when) I will install Ubuntu (or Debian or...
My PC (Esprimo, 3 yeas old) has one hard drive having 2 partitions C: (80 GB NTFS, XP) and D: (120 GB NTFS, empty) and and a 200 MB area that yet is not-partitioned.