Master slave is for IDE drives. SSD's are SATA where there is one drive per cable per connector. 1 drive will give you the biggest speed increase, 2 drives in Raid 0 will give you another speed increase but at increased probability of DATA loss since if one fails the Data is gone.
1 120GB for OS and applications with a HDD for storage is the cost/performance solution.
It is a question of budget but the new Intel 520 SSD for highest reliability followed by Samsung 830, then there are others that are good and cost less OCZ vertex3 and Crucial M4.
1 drive change of drive failure minimal 2 in raid 0 would double that chance.