Hard Disk

I have never owned a computer where I failed to fill the hard disks. Therefore, I am going for an excessive 500 GB for my Vista machine. You need 10 GB of disk space before you can do an in-place upgrade from XP to Vista; upgrading by installing a fresh copy of Vista requires 15 GB. This limitation hits people with, say, 40 GB, disks who formatted their XP machines with multiple partitions of say 10, 15 and 15 GB.
One solution for upgraders is simply to buy a new disk. If you need to do an in-place upgrade, you'll have to use a program such as Norton Ghost or Paragon Hard Disk Manager to copy your old install to the new disk. You'll also have to adjust partition sizes on your new disk, but Hard Disk Manager makes pretty light work of this task.