OK, there are lots of people here insisting that you can simulate a larger machine inside of a smaller machine, in terms of the total state space. Let me assure you that in general, you cannot. That's absolutely a fact. Windows can fool you into thinking it can simulate a 20GB system inside a 10GB system, because it's using the hard drive as the extra 10GB. Which means you really still have a 20GB+ simulating system! I'm talking about a system in which you don't have some other form of memory to fall back on. Data cannot be infinitely compressed, and state space is finite no matter how it's partitioned into dimensions and so forth.
Still simulation means you may not have to represent every particle/state but can instead estimate. Or even not represent them at all until they are observed. Like a graphical system renders only what's in front of the camera. So the effective universe size could be enormous and use fewer resources than is apparent.