openSUSE on 20gb SSD: which party should get a partition on another hd?

Hi,
i want to install openSUSE 12.3 on a 20GB SSD + 500GB non-SSD notebook. My plan is to use the SSD for the OS and the other hd for the rest.
It is clear that i need an own partition for /home but i noticed that 20GB are slightly less than i need for the non /home parts on my current system.
So my question is which parts can i safely put on the SSD (i.e. definitely need less than 20gb ) and what parts should get their own partition on the other hd (what parts do not have to be on the ssd to make my system fast)

And btw is it possible not to have to partition the non ssd? I.e. when i do not need to split up my system i can put everything on one partition (simply be having only a partition for "/") this way i do not have to be absolutely sure about the hd usage of each directory. In my 20/500 configuration above i would put the "/" on the ssd and at least two parts on the other HD but ideally i would put all of the rest on one partition, because i do not exactly know how much of the space each part will need in the end and i do not want to waste space here.