Most major US networks already operate at 3G specs. 3G is just a term used to describe a set of specifications for mobile phone technology. It stands for "Third Generation" and 2G is "Second Generation".
The most noticeable difference at this point on American networks, is data speed - how quickly the internet will work on your phone's browser, how quickly you'll be able to send and receive picture messages, etc. In reality, 3G deals with your phone calls as well, but it's unlikely that you would notice the difference.
One other somewhat noticeable difference is reception. Some carriers have towers that are 3G only, and some frequencies are used only or primarily for 3G communication.
As US phone technology stands right now, if you're just using your phone to make phone calls, you don't really need 3G. If you're doing a lot of data-related stuff, 3G will be nice as long as your carrier has 3G towers in your area.