Which is faster - two 500gig Western Digital hard-drives in a RAID configuration or two single Western Digital hard-drives?
The RAID controller will certainly play a part in any scenario.
If you are using a hardware RAID solution it will be faster to RAID 0 than to have two seperate drives. RAID 1 would be the same or nearly the same speed in most scenarios since controllers are optimized for mirroring out to multiple drives.
Software RAID will always be slower than hardware RAID or just two regular drives. Avoid this at any cost for mission critical usage.
Assuming all the drives involved are the same type: Two drives in a RAID-0 configuration are faster than a single drive, which itself is generally faster than two drives in a RAID-1 configuration.
RAID-0 alternates sectors across each drive, so reads and writes can be sent to both drives at the same time, making it slightly faster. It also has a total capacity of the sum of both drives.
A single drive has only itself to worry about, but has only its capacity.
RAID-1 has all content on both drives, meaning that writes are slower since they have to be explicitly sent to two drives instead of one. However if the RAID is set up correctly, reads can be stepped across drives leading to increased performance. In general though, RAID-1 turns out to be slightly slower than a single, non-raid drive. RAID-1 capacity is only the value of the smallest disk in the array, however the array can continue to function if one of the drives in the array fails. (The "R" in RAID means "redundant".)