partition magic-ally

["prash" <iprash@gmail.com>]

hello,
  i have a spanking new machine with a 40 gb hard disk - all for my
favorite distro! of course too much space is a bad thing too
(especially when you don't know how to allocate it).
  i have decided on this scheme to begin with:
  /               :  7.0 gb
  swap            :  1.5 gb (i have 512mb ram)
  /usr            :  4.0 gb
  /usr/local      : 18.0 gb (i WILL install anything/everything out
there)
  /home           :  4.0 gb
  /home/prash     :  4.0 gb (i am ego(t)istic that way)
  /boot           :  0.5 gb (is this enough?)
  /var            :  1.0 gb (is this enough? this is NOT a mail/news
server)
----------------------------
  total           : 40.0 gb
----------------------------

now, the questions (!):

1. why should i (and how can i) define a /tmp partition when i don't
know what temporary space each app might take? a dvd burner might
decide to take 4 gb, a regular app just 10 kb. if i go higher it's a
waste 95% of the time, lower and i risk some apps not working well.
(this is why i don't have a /tmp defined above: i decided to let the
app take how much ever it wanted out of / (root))

2. suppose i reduce the partition sizes of some of the folders above
and keep aside, let's say, 5gb of "unpartitioned/empty/unused" space.
can i later merge this space with any other partition based on need?
(for example if /usr/local becomes larger than 18 gb - and friends who
are aware of my downloading skills know that that can happen - can i
merge it with this free 5 gb to make it 23 gb?)

3. any other suggestions/rearrangement of my partition scheme?

thanks!
-prash.