37signals recommends 3 people - a developer, a designer, and a sweeper (who can do both).
Assuming everyone can pass the Joel test, I'd suggest (and it's just a guess):
3 Developers:
- A web developer who has build a dynamic website before (team leader).
- A web developer who knows lots about sysadmin (security, database management, project infrastructure, bug tracking, version control, fuzzing, etc).
- A web developer who knows lots about math, statistics, and / or algorithms.
All your developers should know a bit of HTML, HTTP, javascript, and have some idea of what user interface design is all about.
1 developer / designer (who is good at both).
2 Designers:
- An interface designer, who is passionate about user interface design. Business analyst / second in command.
- A designer who has really good design taste. Somebody who can make your site look good.
Hiring a good team leader and interface designer is going to be difficult. A bad lead developer will sink your project. A bad interface designer will make your project suck.