SSDs will load data off a harddrive much faster. Loading windows, games, levels, etc will be faster. Once your in the game they usually run in RAM, so having an SSD won't improve your frame rates.
Modern hdds are fast enough for me. If you have one of the new 500GB platter harddrives you should be good for the most part. I could see getting an SSD for photoshop work or if you are really @n@l about having ultimate speed in windows/games. As I said for most of us a modern mechanical drive is fine.