DASD arm worksheets

Does anyone know if IBM still publishes the "DASD Arms Required for
Performance" type worksheets for the i5 models?  I've used them in the past
as an additional measure of insurance when designing a new system, but the
last time I used one was for an 800.  We're starting to spec out a new 520
to replace the 800 and with the Domino and Websphere workloads I KNOW we've
had disk arm/controller issues in the past, so I want to ensure a good
config.  The 800 was originally spec'ed by the previous admin/BP with a
5702 controlling 6 mirrored drives and the box performed horribly.  I got
it switched over to a RAIDed 2757 and made a positive impact (and want that
to carry over to the new box!).

In addition to the DASD arms worksheet, i'm looking for some input on the
757mb cache and 40mb cache RAID controllers available for the i5s.  We're
hoping to keep this box to just a system enclosure and I think with the
520s all the 757mb controllers are not permitted in the system enclosure.
Is this true?  If so, i'm wondering how a 40mb controller will fare in
comparison to the current 2757 (757mb cache) we're running.  I would hate
to take a step backwards on the disk cache... Domino seems to love the
increased disk cache.