You need a dedicated 7200rpm drive to be up to spec.
Does basic editing perform okay as long as you stay away from intensive tasks like warp stabilizer?
I'm having success with my home system which is similarly equipt to yours.
CS5.5
Core i5 2500k
Raid0 system drives
2TB media drive for all editing material.
16GB memory, Dual Channel
gtx 560ti 1gb
Plays AVCHD fine, up to two streams. CPU load 28% to 38% with 1 stream. Cannon XF codec cuts like butter, no issue up to four streams.