What would I need to get a server running from my house?

I am building a full blown site right now, and when it is finished I will need to have people connecting to it through a server I will have here at my house. It will have to store probably 400 usernames, passwords, and be able to host to all of these people at once. What kind of computer would I have to build to have a server running for those requirements, until the site becomes more serious, and then have to upgrade?What would I need to get a server running from my house?
I would recommend a fast PC, possibly multiprocessor with a hard disk size that would hold the web site. You may need SQL if you want to host databases. You could go Windows, Linux, Mac OS X, Unix, or whatever OS is best for your needs. I would also contact your ISP and pay the money for a business account with a static IP address.What would I need to get a server running from my house?
A basic PC could probably hand that workload. However, your upload bandwidth would be a problem. You'd need to seriously upgrade your bandwidth. You should probably just pay for a webserver. It'd be a lot easier and cheaper.