Interesting question and I have been trying to figure out something similar with my current DB structure and server specs.
My situation is not quite the same as your scenario but may find it interesting non-the-less.
I've got a dedicated server (Specs: Intel Xeon X3330 @ 2.66GHz, 4 GB DDR3 RAM, 500 GB SATA2) where I have all of my sites pulling info from a central DB/Talbe. The table is approximately 1.1 Million records and has 82 Columns of data.
From what I have seen, for my situation, I have noticed that everything runs at 100% between 0-8 requests, 9-15 simultaneous request is a little slower...when I get to around 25-35 requests it gets noticeably slower and things start bogging down....this actually happened this morning - was looking at it and had about 45-55 requests (along with everything else on the server...this is the heaviest resource taking though) and the whole server froze up and had to be re-booted.
Trying to figure out a better set up for myself though as obviously my server crashing is not good thing...