How much hard drive space should I have remaining?

[Firetaffer]

I have a 320GB Hard-drive and have 60GB remaining.

Someone told me 20% of above is usually the best way to go, so should I delete some stuff?

And if I added a new hard-drive how much would it cost?

 

[tecmo34]

Your hard drive will perform better, if you keep 20% or more free. It is really important on SSD's but that doesn't apply for you

I would look to delete items / apps that you are no longer using to clean up space. Also, you can purchase a great 1TB drive for $80 or 500GB drive for $55 (Samsung Spinpoint F3 & Seagate 7200.12) to use as a storage drive or transfer your OS to the newer faster drive.