I'm not saying your numbers are wrong, but...
Those are big numbers. 10,000,000 users a month is 13,000 per hour, 24 hours a day. This site has 11,000,000 members of which only 65,276 are "Online" - i.e. still have current Sessions - at this moment. Very, very little of the site uses HTTPS, none of it deals with money or payment service providers. And it doesn't run on one server! It runs on:
4 web servers running Windows 2008 R2. Each box has between 8 and 16Gb RAM and runs standard hyperthreaded desktop-level intel chips
The web servers connect to heavy duty multi-CPU, mutli-core SQL servers running SQL 2008 R2
Networking is via gigabit NICs
5 email servers
A couple of Active Directory servers
A big backup monstrosity
A handful of switches
About 10km of cabling.
The server rack is warm to the touch.
(See here:
http://www.codeproject.com/info/about.aspx[
^])
And that lot handles a lot less traffic than you are anticipating!
I'd suggest that you need to get a professional in, to look at what you are trying to do, look at your estimates, and suggest a route - because "buying a server" is probably going to be a major mistake!