The "right" answer here is: "it depends". 600 nodes and 2000 edges is not a very large network, depending on what you are trying to show. For example, this image of yeast genetic interactions has ~4000 nodes:
If you are trying to show a pathway and see details of the connections, that's probably too many nodes, and I would recommend simplifying your network. On the other hand, if you have a couple of key nodes that you want to highlight, you can turn off the labels of most of the nodes and then only show the labels of the key nodes after making those larger. You can also tune most of the layouts in Cytoscape. I typically start with Prefuse Force Directed, and then use the Layout Settings dialog to adjust it. Some folks have reported good results with the yFiles layouts also.