That's a very good question!
One thing I learned in my web design class (which was three or so years ago - LOTS change in that time) is that when you consider that particular question, your typical user shouldn't be 'every' single person connected to the internet. What you want to think about is your target audience.
The internet is big - way big, and there are SO many different types of people, systems, and connections involved. What's most important, is what the people your business is targeting as an audience, are using.
For an example: Say you're a Flash driven web-site designer. Your target audience may be very large business, or consumers that can kick the extra cost for DSL or a T connection. They'll be running state-of-the art, or as close as you can come to an industry that changes monthly. They'll have the newest version browser and a high-speed connection. You're not too worried about the fact that the users with a 56k modem have to wait 10 minutes for your Flash movie to load, and even then, it's shaky.
It's all about your target. I work for a not-for-profit membership organization. When we developed our web site, we geared it towards the average to even lower end systems. 28-56k modem speed (we didn't even consider DSL, cable, or ISDN), we were careful about graphics - our links are smallish, there are few scripts written into the page. We tried to take into account that some people would be using Netscape, along with IE. Thing is though, our organization has a very specific purpose - and it was very easy to pinpoint who are target audience was.
Hope that helps some?