You should know that when running a video while extracting files on the same drives is going to knock the fps down a bit knowing the performance impact on the drives. To me those seagates are older drives so I suggest that you buy a new but larger drive for general storage while leaving the rest for work such as your rar files. That is what I do, 1x 640gb, 2x 320gb, 1x 320gb ide, 1x 250gb ide, and 1x 30gb ssd. No need for a optical drive Not when I have others machines to do the job.
Yep. Reading and writing data at the same time on the same drives will cause some stuttering. Additionally, watching HD or x.264 video can be a little stressful on the CPU if it also has to decompress a large file.