I,m wasteful. Through repeated installations, I found that, even with my software installed, my average hard disk use, with Vista or 7, was less than 20GBs.
To allow for contingencies, I gave myself Four partitions of 40Gbs each. I have Vista 64, Windows 7:64 and 32, and, For my grand chldrens visits! XPPro 32bit. On two hard disks I have 320 gbs. Well, not my choice, it came with my main computer.
When my grandchildren visit, they downoad (innocent, I hope!) games. They are, after all computer wise with a top age of 9yrs - lol. These are easily accomodated on all that extra space, where they have another partition for that purpose.
I would be happy to ease out on the space, but I am very image concious, and smaller partitions make for faster backup and restore..
Fwiw. I have never had crossover problems, with partitioning.problems