How can I go about/what is the best way of estimating what kind of server resources I will require at the launch of a new project. Let's say for instance I'd gathered 10k users email addresses that opted-in to be informed when we launch.

I didn't really want to go for a cloud solution at the moment. Should I just get a reasonably-specced dedicated server and go from there, or should I be looking at services like Amazon EC2? Or should I start out with EC2 for the fully-flexible resources, then once things stabilize I'll be able to sort a fixed hosting deal that is adequate elsewhere?

Sub-question: What's a good way of estimating what load a particular application will place on the server per user?