A couple of things:

The 4 GB tuning is a "VIRTUAL" management, not physical.  Each process has a
4 GB virtual address space, that could be served by physical ram or swap
space.  Of that 4 GB, by default, 2 GB is dedicated to Private, or Kernel
mode, space, while the other is for User mode space.  The /3GB switch says
that "EACH" process can now address up to 3 GB User Mode space, but now only
1 GB of Kernel mode, or Private, space.

There is a performance gain for running multiple instances on a single node
for 32-bit architectures in that each instance gets its own User mode
address space from which the Procedure Cache and the other 4 memory managers
are carved, which is limited to the 2 or 3 GB lower memory area.

However, for consolidation and VLDB purposes, Microsoft has shown that a
single instance will typically outperform multiples, unless you begin to run
into this 2-3 GB limitation of the proc cache.  But, then, the decision
should be to upgrade to a 64-bit Itanium platform instead of multiple
instances strictly because each SQL Server per processor license costs you
the same regardless if it is running on an x86 or IA64 cpu.

The following site has tons of information on server consolidation, 32 and
64-bit architecture studies, SQL I/O and SAN information.

Scalability and Very Large Database (VLDB) Resources
http://www.microsoft.com/sql/techinfo/administration/2000/scalability.mspx

Sincerely,

Anthony Thomas

Thank you for the response Geoff

I was mistaken, I actually only have 12 gig of memory on each server.  So in
this case I should allot 5.5 gigs to each server and 1 gig is enough for the
OS?

I should now use the /3GB switch and set the max server memory to 5.5 for
each server.

thanks