You have two easy options...
Easiest and completely free is using VNC.
You can download a program called Chicken of the VNC which is a good VNC viewer for OS X.
You will need to go into system preferences to the sharing setup and enable Apple Remote Desktop, and then click the Access Privlidges button. In there you will see a button to enable VNC Connections.
The second option you have is to get a copy of Apple Remote Desktop 3 (ARD3), which is apple's version of VNC. It has a lot of enhancements over regular VNC. I am actually using it as I am typing this to you to admin my mom's new iMac and help her to get set up. it has a lot of neat features like ability to drag and drop files and folders between systems and they will transfer at your internet speed... you can wake and sleep macs, etc...
Anyways good luck. Send me a private message if you have any questions.