Now that I have installed aperture, can I delete iPhoto?

You could by why would you?

You paid for it, the application doesn;t really take up much space and there may come a time when you want it or need it.

In fact Léonie has wrriten a user tip addressing this see Keep iPhoto - It May Come in Handy!

Picassa is worthless. Almost anything else is better. I use DPhoto instead.

Allan