You eliminate the hard drive bottleneck...system becomes very "snappy", apps load pretty much instantly. I have a couple, and the spinners are just for storage now. Would be a good upgrade for that system.
For gaming, many of the games I've looked at have a minimum of 2gb and a recommended of 4gb. I know that system performance, decreases tremendously when a computer runs out of memory. Having your pagefile on an SSD will help a computer run a lot faster when it runs out of memory, but not as much as having enough memory that you don't run out.
1gb extra isn't much but it is 33% more in a 3gb system, which is a significant percentage, especially if it makes the difference between running out of memory and not running out.
I have a lot of very large software applications, and might need a significantly sized ssd. My adobe folders alone are over 30gb and autodesk is over 20gb, and that is just software, not files.
I'm not saying that an ssd wouldn't be a good investment. If I build a new computer, it will probably be able to handle memory modules bigger than 2gb, rendering any memory I buy now useless or only temporarily useful. An ssd might continue to be useful longer, but it can be a pain to have your programs split between multiple hard drives, which I've done before, and would have to do unless I got a huge, expensive ssd.
tl;dr
Are you sure?
4gb of memory is $20 shipped.
SSDs are expensive.