What will break first?

I’m in the middle of designing a major expansion of our websites.

I’d like to see an example of a image rich WordPress Multisite installation (without HyperDB or SharDB) that can handle tens of thousands of posts shared among 6-10 sub-sites and hundreds of simultaneous users. I plan on having very few videos, but lots of small pictures with links and comments in a gallery layout.

Many of those users will be doing full text searches of content and comments along with filtering by categories, authors, etc. Of course I will have caching and compression and minify and most of the good things that YSlow likes and hopefully no broken plugins or queries.

So where will the bottle neck be? What will break first? Can the standard WP multisite installation with one database handle that kind of site? I mean… is there a limit on the number of PHP processes and MySQL threads that will max out first if I spiked with 10000 users in one hour?!

I learned that I can’t ask here about hosting recommendations. But aside from that, can anybody here give me an example of a configuration like I’m thinking of that works fast and doesn’t break?

Thanks, Ron