I've run a 250,000 pageview/month vBulletin forum (MySQL and PHP, you can judge for yourself how efficient you think this would be as a comparison) off a server destined for the scrapheap:

1.8GHz Celeron, 512 MB RAM, 1 x 5400rpm IDE disk

For both httpd and mysql.

The server occasionally got a bit loaded during peak times (geographically specific market), I'd say I was operating at roughly 80% of capacity, about the edge of comfort.

This was just a hobby site on the side, you may want to consider the additional costs of redundancy, off site data archiving, perhaps even splitting infrastructure across two datacentres and hosting providers just in case.

Ideally, you'd have access to the application in order to run your own capacity testing, can you arrange that?