Right now I think you're worrying about problems that haven't happened
yet.  MySQL is pretty robust for what it does.  Google uses it for a
lot of internal applications stuff and the architecture of MySQL is
improving all the time.  Personally I prefer PostgreSQL  I think it's
cleaner and it makes "more sense" to me.  Yeah, it's had a reputation
as a bit of a dog but if you haven't looked at Postgres 8 benchmarks
you owe it to yourself to see how PostgreSQL is doing now that the
code base is growing up.  A lot of time and energy is now going into
optimizing performance these days and it has made a real difference.

If you want to be blunt about it, given near infinite resources and
significant database complexity, neither MySQL nor PostgreSQL can
touch Oracle.  However, given finite resources and reasonably simple
DB structure with lots of brief OLTP queries instead of huge long
linear ones, MySQL and PostgreSQL can perform fairly close to on par
with Oracle but all talk about database speed needs to be prefaced
with a caveat that your DB structure and code, your hardware and
storage subsystems and how you have tweaked your chosen DB for
performance for YOUR application will all play a more significant role
in your overall application/transaction speed than which database you
chose to begin with.

For a mere "thousands of transactions per minute" any of the discussed
databases will work depending on how complex said transaction is.  My
advice is to write the best SQL you can that as is as close to ANSI
standards as possible (See Joe Celko's excellent books on this subject
for more info), pick one or the other of the free options (I'd go with
PostgreSQL for constraints and foreign keys alone but hey that's MY
preference and comfort zone go with what feels right to you) with the
understanding that if this really takes off, you'll be planning a
migration to Oracle (which happens all the time) if you find yourself
in a situation where the database you started with is now
underperforming.  Just make sure it really is the DB software that is
causing your limitations, not your hardware, storage, or your code (or
any number of middleware solutions for ORM) that is the choke point.

Good luck!

Micah
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