This semester we will be using Adobe Illustrator extensively. There is no requirement to purchase the software -- there is class time and access to the software in the GGS Reasearch 1 lab (which is open 24/hours). But....many students find it helpful to have the program at home or on their laptop.
The current version of Illustrator is CS6 (http://www.adobe.com/products/illustrator.edu.html ) but their suite of software is often a better deal. Student discounts are very good deal with Adobe. Here is the campus computer store's pricing: http://compstore.gmu.edu/products/adobe/
One more thing: there is a FREE 30 day trial offered with every Adobe program. This is very handy, but you cannot reuse the trial. So you can opt to use it or wait until 30 days before the final project is due....