I don't know of a database installation (even Oracle) that can
handle that # of requests on a single instance. I'd be happy to
hear about someone who has seen this, but that kind of peak
load is bigger than anything I've even heard of.
There at least one immediate potential solution which, by the
way, doesn't apply specifically to open-source. You can partition
your data. If there's no "collaboration," you may be able to
segment your user base into multiple systems, literally running
separate database instances, and routing everything through a
single first layer so everyone can start from the same point.
