A SSD will make your system feel much faster/responsive, but it will not generally increase performance in any Graphics or Processor intensive tasks.
One possible exception could be if you do not have enough RAM available and are constantly hammering your page file; but this is a highly unlikely scenario for the vast majority of consumers.
That said, if you can find the budget for a SSD, I would highly recommend adding one to your system.
Coming from a standard HDD, you will not be disappointed with how much snappier your system feels.