You might need a special mounting bracket. (they are cheap) Obviously the SSD is smaller than the HDD so you need something to keep it in place.
Also, it is best to do a fresh install of your OS. Trying to transfer OS and programs from HDD to SSD is a nightmare. When you do decide to do fresh install of OS, do not plug in your HDD. Install OS on SSD then restart with HDD plugged in. Also, make sure you enable AHCI in Bios.
Yes! It will speed up loading times, the time from you clicking the program to the time of open window, something like this, however SSD provides little assistance in games, unfortunately.