When you sign up for a host, what is difference between a dedicated ip address and a non dedicated ip performanc or feature wise?

I know a dedicated is like http://66........etc and a non dedicated would be like http://something.com/~something

Besides the dedicated being a top level and the non being a sub domain is there any difference that would affect my site?

A dedicated IP address is usually only required for certain features such as your own SSL certificate, anonymous FTP, etc. If you don't need these features at the moment then a shared IP address will be fine for you :)