Must remember ONLY reads and writes are speeded up by a SSD, they do not make programs run any faster, ie for games your FPS will NOT improve - Only the time to load the game and periodically when a new "map" is loaded. So Yes you can buy a SMALL SSD and just put the game on it.
Most do as WR2 stated "..Win7 and your most frequently used, or most important apps on the SSD."
This provides very fast Boot time and program load times, for programs on the SSD. And Here it Does NOT improve download speed, nor How "fast" the program runs after it is loaded into memory. Extreme example - Microsoft office will load in less than 1 sec but How fast it runs is limited by your Typing speed!.