If you don't need that much space, why not pick up just two Samsung Spinpoint F4 320 GB drives? They are faster than the 1TB drives and cost less. It will give you 640GB of space in RAID0 and keep your 1.5 TB drive for any required overflow storage.
To get true speed, the OCZ Vertex 2 is the way to go. It will out perform even a RAID setup of the Samsung's. I would recommend saving up for one down the road and have it replace your 1.5 TB setup and run with the SSD and RAID0 of F4's.