Basically you can set up a wireless network between your two computers. However, sharing an Internet connection this way is a major pain, and by the nature of this computer-to-computer network (they're called "ad hoc" networks) you have to set it up every time you connect. Getting a router will make your life a lot easier in this.
I think you should get a wireless router and set up a permanent (or "infrastructure") network. These boxes are inexpensive, fairly easy to set up, and they make sharing yoru connection MUCH easier. You do not need an AirPort Base Station to network your Macs, though that's one choice. ANY product that supports the IEEE 802.11g/b standard, which is also known as "WiFi," will work for you. Plus, most third party wireless routers have four wired ports, increasing your flexibility.