Are they moving because your current shared host says you have to? Or they want the security of a dedicated server?
There are so many variables in how much power you need it is really hard to recommend without a decent knowledge of the current setup, for example the serving of 50,000,000 static small html pages, although it sounds a lot could be easily taken care of by a low end server, however a badly written bit of dotnet or php calling a database a few times can easily take down the most powerful server.
Your best bet might be to go and talk to a few of the larger hosts out there, explain what you are doing and let them recommend a system. Get a few quotes and see who you fancy. Remember in the hosting game cheaper is not usually better!
Your best bet might be to go and talk to a few of the larger hosts out there, explain what you are doing and let them recommend a system.
I wouldn't do this to be quite frankly. I have done some systematic searching among all larger hosting companies in the US a few months ago to find a dedicated server for a specific project. I systematically asked the same questions and gave the same information, including:
The site doesn't need much processing power, a Celeron type CPU will do
X amount of RAM needed where X wasn't a real high figure
Y amount of bandwidth needed where Y wasn't very high either
I cannot count the number of offers I got for multi-core, multi-processor systems, and one even adviced me a 1/3 height rack space with three servers in it, just to be sure if one of the servers failed.
My conclusion was that in hosting business sales people sell what they have available and which gives them the highest margins, not what you need for your situation. Determine for yourself what you need and look for a decent host offering just that. If you really don't know what type of configuration you need, you shouldn't go for a dedicated server anyway and stay on a shared configuration where the hosting company takes care of maintenance.