Partitioning is as much art as is it science. I have faced this question a few times and have yet to find a definitive answer
One thing to consider is keeping the root and home partitions separated as this (is supposed to) make upgrading easier. 160Gb is quite a bit for a Linux system and unless you have A LOT of data that you want to make available, I doubt you will want for space. Since you have two identical drives, I would consider setting them up as a raid configuration with the drives mirroring each other. If you don't put /var and /opt in their own separate partitions, they will allocate the space as they need, up to your capacity. Unless you have a good idea in advance of how much each will require, I wouldn't assign fixed sizes.
