My only advice is to hire a consultant who can give you some advice. If your site is getting taken out by 50-100 requests per second (that is a very low number)

My only advice is to hire a consultant who can give you some advice. If your site is getting taken out by 50-100 requests per second (that is a very low number), you have some basic structural flaws. A few hours of time from a developer experienced with scaling will save you a lot of grief. Specifically, it sounds like you need a caching strategy (probably for all layers of your app, including the database). Super-general overview: Use memcached in front of your DB. Use a reverse proxy like Varnish and/or a CDN in front of your entire site. I don't want to make the obvious guess that this is a PHP site, but if it is, make sure you are using apc or xcache as well.