Depends if you have the Go, the Go Baby or the Studio. Not all dies fit all machines. I have a Baby and have the basic geometric shapes and a couple of "fancy" dies that I got used at a quilt show.
Figure out what type of quilts you make and go from there. Accuquilt has a terrific website and there is a chart of all the dies they make, which size mats to use with the dies and which machines they fit.
You can get some good deals on eBay on dies, but there are some listed that are way, way, way above accquilt's suggested price.
I love my Go Baby. Hope you enjoy your Go.