>I've heard that it is important to disable disk caching on the faster drives. Is that correct?

Where did you hear that? Disabling disk cache WILL decrease performance A LOT. That defeats the purpose of 15K drives.

>We're having difficulty turning off the cache on one of the machines and our hardware guy wants to make sure this is what we want.

And rightfully so. Bear in mind that sometimes high-performance drives are attached to controllers that implement a cache in hardware, and which can't be turned off.