Aperture and iPhoto now share the same unified library format, see:
So you can simply launch Aperture on your existing iPhoto Library - select the library in the Finder, ctrl-click it; from the pop-up menu select "Open with" and set it to "Aperture".
I would not delete the iPhoto Application however. It might come in handy one day. Only iPhoto can show you the managed original image files inside the library package, or you may want to view one of your older books and other print products. You may accidentally have hidden an image in iPhoto and need iPhoto to reveal it again.
The PhotoStream is tied to the library, and you will be able to use it with Aperture as well.
I moved from iPhoto to Aperture and did not regret it. I found the Aperture developement much more conservative than for iPhoto, but that is my personal view.