RAM is used for meshing. 8GB should be plenty unless you are getting into some very complex studies. Obviously more is better but if you have to pick I think the SSD would give better computer performance overall, not just with Solidworks. The simulation solver uses the processors and hard drive so an SSD is going to speed up intermediate result calculations.
Here is some light reading related to your question also:
have you looked at the solidworks kb? there are some great articles about performance in soldiworks simulation, which solvers use multi core...etc
but what it comes down to is this, if your analysis is optimized
RAM > add more if you are going into page file
CPU > will help to a certain point on solve time
SSD > is probably the biggest help if you are using a spinning disk today
BUT all of these are based on an optimized analysis setup and making sure that you're doing what is right to reach the goal of your analysis. for example if you have slow solve times and are trying to use solids for a part that should use shells, throwing hardware is a waste of your time. setup the analysis with the right idealization and you will speed up your simulation significantly. in our consulting business here at Hawk Ridge Systems, we solve almost every problem on a computer that is about the same level as what you have right now.