Is there a limit on amount of space you use, until your imac begins to slow a bit? I heard that I should stop when I use up 80% of my hard drive. I'm at 500 gigs, half way, I'm just curious, because between my RIPIT use & my WOW use, my GTA anthology use, & PCSX use, space goes a bit rapidly.

There is definitely a point when things start to slow down... but for me I've found it is in the last 5GB of remaining space that causes slow down. (Ive had drives ranging from 100GB to 500GB). You should give yourself a good amount of buffer space and not let things get that bad.

Oh, and back up often!! Hard drives fail... replaced one on my MacBook this year.