This simulator is absolutely awesome. It requires that you have the Cisco IOS images as it comes with a MIPS emulator and actually emulates a real Cisco router, switch, or PIX firewall. It's so realistic that you can design a network, configure the routers and switches, and then drop the running configurations onto real network gear.
It certainly helps to have plenty of RAM available to run this. 1 GB or more is almost a necessity.