Spécifications / précautions that we need to think about for a highly visited website

I have mostly worked on moderately visited websites, say about 5 - 10 thousands uniques a month - and standard php / mysql codes have worked very good for them till now.

There’s a possibility that I may have to work on a project where the website is getting 10 million + uniques a month and they aim to get to 20 million uniques in next 2 / 3 years. I was wondering what precaution (if any) I should take because of the high uniques. Also, what I would do if I were to make that site from scratch ?

For the database, for example, will the mysql be enough or I should think about Oracle straigt away. Or if any special précaution that I need for site / server security for the high volume.

Any tips welcome. Cheers.