As part of a business strategy, I'm contemplating offering free access to some parts of my website which typically are only accessible to paid users. Because of the specific market that I'm in, this could quickly bump up the number of unique users to my site from a few thousand to tens of thousands. Assuming that:

1. My site uses a CDN for images, css, and javascript and I've combined/optimized css/javascript.
2. Each of the pages is processed using php, but for the most part are "static" due to caching of relevant database queries
3. If a particular page has video, it would be embedded and provided by another service such as Youtube
4. Each unique user will view 3-4 pages of my site per day

Is there any way to even guestimate the level of hosting that would be needed if I have:

a) 50,000 unique users per day
b) 100,000 unique users per day

Could I get away with semi-dedicated shared hosting? Do I need a VPS? Or, would it be prudent to move to the cloud and implement load balancing?

I hope that this is enough information to get a sense of my best strategy.