Before finalizing your decision about a memory card, have a thorough search on which technology has the easiest interface support for the arduino. Ideally, I think you want a library that allows you to read and write data as if it was an array of bytes.
Variable length strings are awkward to manage in a file system and if you use fixed length strings, you might as well use fixed length binary data. My suggestion is you have each data 'sample' as a two byte pair, the first is the servo number and the second byte is the servo position where 0 and 255 represent the extremes of movement. To start, it could make things simple if you treated the SD card as a two dimensional array, where one dimension is the servo number (from 1 to 8) and the other dimension is the sequential value of the servo for all the samples you have taken.
Once you figure out how you will do the data logging you may want to look a some of the nunchuck interfaces published. The
video here gives an idea of what can be done and the interfacing of published code is not all that difficult.