Well, rather than address any questions of reliability and such, I thought I'd try to answer the technical aspects of your question.
So what types of web services are you thinking of hosting?
For instance, if you are thinking of hosting ReSTful web services, what are you thinking of using to implement your services? If you want to implement in PHP, Perl, Python or Ruby on Rails, a standard Dreamhost account may be sufficient depending on your load requirements. If you think you'll require dedicated memory or CPU, or if you're going to implement in Java/Tomcat/Resin or the like, you'll need to go with Dreamhost PS. I say that because all Java application servers require a persistent server process and you need a Dreamhost PS account to run a persistent server process.
I'm not quite sure what one needs to run a .NET application server, but I have a suspicion that if that is your intention, you might be better off using a web host running full .NET capable servers rather than try to run .NET under linux. This isn't my area of expertise, though so I'll let others provide their better opinions if that's what you need.
Let us know more about what you want to run and we'll be able to provide better answers.
Disclaimer: I'm just a happy Dreamhost customer. I use Dreamhost PS for my forums, blogs, and such but I don't actually run any web services on Dreamhost, though I work for a company that does build enterprise-level web services and web applications in financial services.
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