I make a habit of creating a " STORAGE" folder on my desktop.
I save each installer .dmg and the corresponding authorization numbers in "STORAGE" until the folder is large enough to burn to CD. ( about once a month )
I also save copies of all the Apple software update .pkg's in the event I need to reconfigure a drive later.
I have an insane amount of stuff on my system and it still only totals 48 GB on a 250 GB drive.
I also have another 160 GB storage and maintenance drive with back-ups of everything important.
I re-installed the OS as a lean install of 7.4 GB by custom deleting
all the extra language translators, printer drivers, software demos,
iDVD themes and GarageBand demo songs.
The default Mac OSX install is about 12.5 GB. plus formatting.
If you control click " get info" on your hard drive icon, it will tell you how much free space you have available on your hard drive.
You can free up about 5 GB+++ of valuable HD space running a custom lean install.
That's enough space to install some serious pro applications and still leave you plenty of room on the smallest notebook drive.