Yet another postgres scaling question (use on AMD Opteron)

Greetings,

I am looking for feedback on the AMD Opteron based platform as a server
for Postgres, and also comments.  The databases I am looking at will
likely have aggregate sums of indexes larger than the amount of memory on
a 32 bit machine, so I am looking at 64-bit platforms.

Does anyone here have experience with use of the Opteron platform?  There
was a post a while ago by someone asking if it would run on a Beowolf
cluster of opterons ( indicated it won't ) but I am interested in knowing
if anyone has actually implemented it on an Opteron machine.  It should
work since Opteron runs linux, but knowing that it _is_ in production
somewhere would give me more leverage to get an implementation plan
approved.  I know that many here have used Sun and IBM to scale postgres
but looking at the cost, Opteron is the most attractive right now.

The database sizes I am looking at are in the realm of 5-50 gigabytes.  So
far I have been able to achieve acceptable speeds on a 2 gigabyte database
with a 32 bit machine, but as the data size grows an argument has been
introduced that going with MySQL would allow us to scale with smaller
machines (hold the flames please :).  I have optimized and tuned the
hardware and database according to the fine advice given on these lists
(especially according to the kernel cache/shared buffer cache balancing
threads).

Comments and suggestions greatly appreciated.

Thanks

Fred Moyer