Just to stir things up a bit...

It has been my (recent) expierence that Solaris 8
on a 900-MHz Intel box will (in general) run faster than
Solaris 8 on a 464-MHz Sparc box. Having said that,
in an SMP configuration, Sparc's "tend" to perform better.
Add the server attached storage (I assuming you are not
using a SAN configuration), now you're dealing with
periferal I/O as well. The "true" way to tell is to set them
both up and run some benchmarks. You may also see some
differences due to the 8MB vs. 2MB cache. I suspect that
unless there are substantial differences between the A5100's
and trhe 4100's, you may not see much difference unless
Solaris on Intel SMP shows some big problems.