Unfortunately, real, unbiased reviews of multiple RDBMSs are hard to come by.
Part of the problem is that a substantial portion of the performance of the DB is/can be tied to the underlying O/S and hardware platform.
MySQL is fine if your hits are going to be mostly select statements - it's heavily optimized for this.
PostgreSQL is often better when you are doing more write type transactions, since PG supports, among other things, transactions (reason enough to go with PG, it's a VERY useful feature!)
I've heard a fair amount about MsSQL, most of it bad, but I've never developed on it, worked with it, or paid much attention since I devel on Linux, so I really don't have much to say there.
And that is part of the problem. How many people can really compare Oracle against Postgres, Oracle against MsSQL, Postgres against MsSQL, against MySQL, on all the different platforms, and actually come up with an "unbiased" opinion?
If it were my call, I'd recommend MySQL if you are doing mostly reads, (select ...) and either PostgreSQL or Oracle ($$$) if you are doing more writes (insert into... update...)
MySQL, from what I've seen, is the DB best integrated into PHP.
Oh, and if you are serious about the loads you plan on working under, use Unix. Windows just doesn't handle the load as well and is not as scalable as a well-tuned *nix.
-Ben