Raid and I presume your talking about Raid-0 will mainly increase your sequential speeds and your random data transfers. Sequential write speed and access time are pretty important in my opinion for what you are doing.
Raid-0 does improve sequential speeds but does not help access times. More importantly if one drive fails all of your data is lost.
Raid-0 mechanical drives would not make sense compared to a SSD as the SSD is usually faster and raid performance can be dependent on your SATA chipset; if I remember correctly there was a bug in the Intel chipset used in the P67 boards.
Intel have a program that can check for the flaw, but I will presume you do not have it.
Raid would not really be worth it in my opinion.
Hard drives have platters and the density of the platter influence performance, reliability and cost.
A 1tb platter density is the latest at the moment. Basically your fitting more data in the same amount of space.
So as your hard drive is spinning a 7200rpm (for example) it can effectively get to the data faster than a hard drive with a lower density as it is a lot closer together. The only downside is that access times increase.
The less platters you have the less you have to fail and again the less platters you have to include the cheaper it becomes for the consumer.
The WD Black was the fastest consumer drive that WD made until the WD10EZEX came out, or the 1TB WD Blue. The Black would be worth considering due to a 5 year warranty opposed to a 3 year warranty of the Blue.
The more platters you have the more performance you can get. A drive with 2, 1TB platters would outperform a drive with 1, 1TB platter.