Grab yourself a G-Tech G Safe which is 7,200RPM all the time, RAID 1 Redundancy and built to take the rigors of constant seek/read/write… 1TB@ $350, 2TB @ $550 or better 3TB @ $700.
How much space do you have in original files? Are you planning to edit natively in Adobe Premiere Pro CS 5.5? The more DIMM you have and the better the GPU the better all around. Make sure your Mac is current for OS, firmware and software upgrades and turn off everything running except for your Editing software.
Are you editing on a copy of your original files? Hope so. If you go to CS5.5 you can edit natively without transcoding, but your machine needs to be 64-bit architecture.
Good luck and happy post(ing).
Jon Frost