What size of drive do you have in the computer at the moment?
The simplest option on your end would be to buy yourself an external drive, and then move across all the files you won't need regularly - so all the archived drag strip photos that won't be needed in the near future but you want kept, and you could shift a lot of the movies over and either just view them from the external drive, or copy a few back if you are going to be away and want to have something to watch with you.
The slightly fancier option would be to upgrade the hard drive - if you are running a 120gb drive or similar, you could get yourself a 320gb or 500gb drive for fairly cheap nowadays, swap that with the drive in the mac and copy your data back over to the mac.
A fairly simple swap, but if you are not comfortable doing it yourself it will be an easy job for a tech to do.
As a bonus, you would still have your current drive, so spend an extra $15 on an external case for it and you have a spare drive which you can use for backing up important files or for moving data between computers or whatnot.