I have a 15" 2.53ghz, 4gb, 320gb HDD MBP. I plan on storing music for itunes, videos to dabble in video editing and tons of pics for photoshop and web design. At one point is it healthy to start storing assets on an external HDD? 50%, 60%, 75% of HDD capacity in order for your system to run smoothly? I hope my question is clear. Thanks!