Here is a list of criterias you have to gather to get an accurate assessment of your technology needs:

• OS: if you use ASP.NET or MS SQL Server, you obviously need a Windows Dedicated Server. If you use an open source stack such as Apache/php/mysql or Ruby On Rails, a Linux server is the best choice.
• Web Server and Database Server: depending on your technology stack, you might need a server with plenty of RAM, especially if the architecture builds upon Windows, Java or Ruby.
• Performance: for SQL servers, video-transcoding severs, servers used for virtualization, or specialized servers such as game or chat servers, you need fast, multi-core processors or ideally multi-processors servers, such as dual xeon servers.
• Bandwidth: if you plan to host images, multimedia files such as flash videos, or an Asterisk server, bandwidth provided by the hosting provider is a key criteria
• Management: if you lack expert sys admins at hand to manage your server, you should look at advanced managed services. A good web hosting company should be able to offer you expert advice on how to do load balancing or web clusters, as well as good support.
• Network: depending on the audience and geographical location of your web service, you should look closely at the location of the data center.

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