Hard drive full or near full:
You are going to have to delete, move or archive your data to regain at least 15-20 GBs of empty hard drive space.
Have you emptied your iMac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Other things you can do to gain space.
Delete any old or no longer needed emails and/or archive older emails you want to save.
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive or delete any old documents you no longer use or immediately need.
Uninstall apps that you no longer use. If the app has a dedicated uninstaller, use it to completely uninstall the app. If the app has no uninstaller, then just drag it to the OS X Trash icon and empty the Trash.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
You may have to Purchase an external FireWire or Thunderbolt hard drive to move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.
If you have any other large folders of personal data or projects, these should be moved, also, to the external hard drive and then either archived to disc and/or deleted off your internal hard drive.
Since cache is there to speed up the system, deleting it usually slows down the system until cache is regenerated again.
It has been an old wive's tale that it speed things up but that is not true.
The only time that deleting cache is valid is when it gets corrupted. That is usually an extremely rare event so this clearing of cache for any and all problems is a bit overplayed.