When RAM runs out on a computer, the swap file is used, and then the system can become quite slow. But if the notebook or desktop is using SSD nowadays, will it be less of a concern?
(Updated Oct 2014: For example, if we now buy a notebook computer with SSD, would it make sense to save some money not to buy more RAM, such as staying with 8 GB and not go for 16 GB, because SSD already make the swap file issue not a big issue?)
The best you'll get out of an HDD right now is 150 MB/s.
The maximum speed of transfer of an SSD is 600 MB/s, as that is the max speed of a SATA3 connection. They are usually slower than this though.
The maximum speed of transfer of DDR3-800 RAM is 6400 MB/s (so the slowest DDR3 RAM around)
These are all approximate and handwavy. However, while an SSD is quicker, HDD is 2% of RAM transfer rate, and SSD at its very best is 9% of RAM transfer rate.
So "it won't slow down the system so much", but whether you'll notice is another matter. More RAM is the answer to running out of RAM, not faster a hard disk.
