SSD & Video editing

SSD is associated with super high speed & performance but in what applications? Will it improve HD video editing if I add in an SSD to my system? My system is P55/Intel Core I7 870/ 8Gb DDR3 1600 RAM/Nvidia GTZ 570 VGA/Corsair 850W PSU/4 X 2TB HDD/Win 7 Ult 64bit/Avid Studio optimised for 64bit.
Currently when doing HD editing, my CPU usage top at 55% & RAM usage less than 3Gb. Average fps is 20 - 30 during rendering to a bluray.
Which type of SSD is better - Sata or PCI express interface?

How large are the video files? Do you know if your software has been optimized for multi-core cpu’s?