RAID arrays and performance

I have a question about how Postgres makes use of RAID arrays for
performance, because we are considering buying a 12-disc array for
performance reasons. I'm interested in how the performance scales with the
number of discs in the array.

Now, I know that for an OLTP workload (in other words, lots of small
parallel random accesses), the performance will scale almost with the
number of discs. However, I'm more interested in the performance of
individual queries, particularly those where Postgres has to do an index
scan, which will result in a single query performing lots of random
accesses to the disc system. Theoretically, this *can* scale with the
number of discs too - my question is does it?

Does Postgres issue requests to each random access in turn, waiting for
each one to complete before issuing the next request (in which case the
performance will not exceed that of a single disc), or does it use some
clever asynchronous access method to send a queue of random access
requests to the OS that can be distributed among the available discs?

Any knowledgable answers or benchmark proof would be appreciated,

Matthew

--
"To err is human; to really louse things up requires root
privileges." -- Alexander Pope, slightly paraphrased