Autologin feature killing my website
You could cache the content and displaying it cached would ideally take stress off your database. Check out module "Boost" it can help.

Ideally optimization of the site is what is needed along with better hardware and maybe Varnish.

If you are using views consider using views built in cache with time based expiration to take stress off database. 

Update

If you already have Varnish, APC and memcache then the only other think you can really do is get some better hardware. 

We have servers that get about double what you have and they run on a quad core processor with 32GB of RAM. 

We also split Web and DB server so we can utilize APC for Web Server and memcache for DB server. 

Have you checked your APC stats? Varnish probably won't do much unless you configure it to cache Authenticated traffic which by default bypasses Varnish. 

What are your server stats?