I used to do a lot of capacity planning. The actual hardest part was persuading t' management that:
a) I knew what I was talking about
b) that they would have to spend money
However, the traditional process as described in ITIL and ISO9000 has largely become obsolete. What seems to happen more is that systems become vulnerable to unpredicted and often unknown behaviours due to crappy software design and architectures. Oh .... and networks.
Things that cannot be solved simply by running down to PC World for all the SIMMs they have in stock, or invoking the magic of capacity on demand.