I think some of the obvious things you need to consider are clustering, at the database level and at the doc lib/image lib level.
You should also be fronting w/ some sort of static file handler at the http server level (to serve up scripts, images, css, and other static content) to lighten the load off of the application server.
I'd avoid MSSql simply because of the issues w/ it. I've had lots of luck with Oracle, but the pricetag is hard to swallow.
I'd prefer a unix OS, simply because it is easier to harden, but I'm sure you'd hear differently from the MS camp.
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