Setting aside Server setup for the moment and looking at a default installation, personal experience has shown that PostNuke is OK, in my books. Haven't experienced the load factors you mention, but cannot see where there would be too much problem -- odds are the load factor will not be as high as you stated.
Experience will be your best guide so watch carefully during the first month. If the load gets as high as you stated and stays consistant -- Server setup would matter more than which Forum script was being used -- it would probably be best to move to a Dedicated Server for the account.
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Regardless of Server specs (and going with as much RAM and CPU speed as you can afford), tuning a database is basically managing memory and juggling with available allocations across; indexes, buffers, log files, archive logs, rollback segments etc.,
There is not any one answer for optimizing. How well a script or database will operate depends on the; type of and number of sites on the Server, plus the intensity of scripts being used by all.
Few months ago I made some benchmark tests with postnuke on my celeron 700Mhz (128MB RAM) redhat box. I've tested plain postnuke installation without any extra modules and was able to server 3 pageviews per second and with phpa installed (php caching engine) I was able to increase it to 8 per sec. Teoretically, that's over 600,000 pageviews daily but I think that with 100 concurrent users you would sure need something more than simple shared hosting account.