If your HDD is nearing full capacity, here are tips to free up space without installing a new hard drive:
I'm sure most of you know this, but I thought I'd post it anyways.
Also: You would think that PSN games take up the most space on you HDD. In fact, the game data takes up the most space. Music, photos, trailers, demos, PSN games don't really take up that much space, unless in large amounts. I never knew this until I looked in my Game Data Utility. A lot of your HDD space goes here. Each file contains about 3 gb of data. On my PS3, there is about 35gb in this folder. Don't be apprehensive about deleting stuff here because it will reinstall the next time you play the game, so if you don't play a game very often, you can delete the game data to free up a bunch of space.
Seriously. Just upgrade the HD. It takes 5 minutes. It is the best thing I have ever done for my PS3. I had a 40GB drive that I ran out of space a few times. It's a pain in the rear when you dont want to scrap anything but you must.
You can get a 320GB drive from newegg for less than $50