Buy a portable second HD. Transfer your chosen files to it. Check that they are safely transferred. Then clear what you can from the internal HD.
The advantage of a portable, say 1Tb, is that you can store it (and its valuable contents) away from the machine, indeed, away from the house and therefore protect the contents against fire, theft, lightning strikes and whatever. I use such an HD; my entire system is on it and I top up every couple of months or so.
WD have a range of such portable HDs and I'm sure there are many others. Ideally it should be about 2 or 3 times the capacity of your internal HD. And continue to use Time Machine. What arrangements do you have for that?
I have three external HDs on my iMac. I run Aperture and all my image files are on the 'main' external HD and they are all backed up on a 2nd identical HD. The 3rd is used to back up my iTunes files and my Aperture Library. Right now the Aperture Library is located on the main external HD and I would like to move it back to my internal HD for better performance.
One route would be to move all my iTunes files off the internal HD on the the #3 external HD to free up about 50GB.
Yes, the iTunes files and iPhoto are the obvious targets. You could also consider buying a big capacity external HD and running your computer directly from that.
I don't think it would be worth considering buying a new Mac if the current one is working well. But if you did, you must first check that your apps will function with the totally different system that Mountain Lion (Mavericks) uses.
I could move my entire iTunes library to the 1TB external drive and set my Time Capsule up to back it up along with my internal HD. That way my iTunes library would be protected. That would free up about 90GBs on my internal HD.
You can only add an extra one by removing the optical drive & using an adaptor to hold another HD in its place.
See http://www.ifixit.com/Device/iMac_Intel_24%22_EMC_2134_and_2211 for instructions on that & the standard replacement.
Hello again, dsloan,
Always comes over so impersonal, the user name, but never mind.
That 90GB sounds good and much better than your initial forecasts. I do hope all works well for you; thank you for the star.