I think I would put the /home directory on the SSD, but for most directories, especially the ones with large files, I would simply have a symbolic link to whatever contains the files.
This way, you can keep your settings, and the browser cache, etc on the SSD, but put the pictures, sounds, documents, etc on whatever disk you want, which can easily be changed by changing the links.
To do a backup, you can usually choose to follow symbolic links if desired. I use sbackup (simple backup), and have separate profiles for major directories, and so each profile can choose whether to follow symbolic links or not.
Edit:
By the way, a slightly more complex and harder to maintain, but possibly easier to use method (in that you don't have to pay attention to where programs save files by default) would be to mount the various documents or media folders or partitions in the /home subdirectories, so that programs that automatically want to save pictures in /home/Pictures will save them there, but they will get saved to the external location.
Putting / in the SSD and Swap and /home in the HDD seems reasonable.
/ will contain the OS, all the applications, and any server related stuff that does not belong to a specific user. For example, if you run a web server all the data files for the server will be in /.
So how big / should be depends on what you plan to do with the box. For "normal" desktop use 15-30GB should be more than enough. This should be sufficient for the OS and all the applications you may install in the future.
The advantage of keeping the OS and applications in the SSD is speed. However, personal settings and preferences are stored in the /home. So for the fastest results you may want to keep the /home in the SSD as well and use symbolic links to the folders with large data, such as Documents, Pictures, Music, Videos, etc. See Setup for dual disk (SSD+HDD) with /home partition for some details.
Hope this helps
Best Answer
Having certain HOME directories located on the SSD will considerably speed up your overall system performance.
However I reverted that leaving all subdirectories in their standard location on my HOME on a hard drive simply because backups and restores are easier then. Also in case you break something and will have to re-install your OS you will have to take extra care not to overwrite your symlinked directories on the SSD.
In case your SSD is big enough to hold most of HOME (which seems to be so in your case) you may consider putting less frequently used directories or those containing the big files on a separate hard drive and link e.g.
~/Musicor~/Videosto the hard drive location.As the often heard agument that frequent write cycles to the SSD may wear it out earlier may be true but we do have to keep in mind that it also are just those frequent things we may want to have speeded up (then at the cost of the lifetime of our SSD).