SSDs only speed up disk I/O. All games use disk I/O when they first load, so SSDs will speed up the load time. After that, it depends entirely on the game and on how much RAM you have.
If the entire game doesn't fit into RAM, then the system will swap memory out to the pagefile. If you're in that situation, having the pagefile on an SSD will speed things up, but a much better solution would be to add more RAM (bearing in mind that if you go to 4GB or beyond you need to use a 64-bit OS to be able to use it all).
If the entire game does fit in RAM, then an SSD won't make any difference to the actual game play unless the game needs to read from the disk. In some games this might occur at scene changes, for example.
The way to tell is to play your game and if it seems to pause or stutter then pay attention to whether or not the disk activity light is always on a lot at the same time. A flicker or two of the light doesn't mean much, but for example if it's on pretty solidly for a few seconds then an SSD could potentially help.