Requirements on memory and disk sizes depends on the workload, but at least 512MB RAM and several GB of free disk is recommended. In general, it helps to have at least as much memory as what is being used on the simulated machine to avoid unnecessary swapping. A fast disk system speeds up loading and saving of disk images and checkpoints.
If you are running Simics on a laptop and want to reduce the power consumption, you can try to use the enable-real-time-mode command to prevent Simics from simulating faster than real time (see the Simics Reference Manual for more information on this command). To further reduce the CPU usage of Simics, you can pass an argument to enable-real-time-mode that is less than 100 (in percent of real time).