Of course, it depends entirely on what you want to do.
If you want to do heavier sim-stuff, get as much ram as you can - in my opinion 16MB is the minimum for sims.
For Win and Linux, you should look for a model with a at least mid-range nvidia graphic card. Not sure about the driver situation on MacOS , though. Most people recommend linux for running houdini.
Intel cpus seem to be the right choice. For Mantra and dops, get as much cores as you can.
Working on a system with SSDs feels much faster. But in my cases I didn't notice a *big* difference while modelling or rendering.
Usability is another aspect to consider. So, the higher the resolution of your display, the better it is.
And: You need a middle-mouse-button in houdini!