Well that depends on a few things, Do you find yourself wishing that you could boot in sub 15 secounds? Do you find yourself being annoyed by quick lil fps drops in games when loading a new area all be it not really that noticable can be slightly annoying, waiting to maps to load in new games? Becuase if you do then an SSD can be very beneficial. But make sure that you have a SATA 6gbs connection becuase if your running a SATA 3gbs port on your motherboard, you will only recieve half the sequential read and write(load and write speed) making them not worth the money, IMO. If I were you and this only applies if your considering buying an SSD, I would Buy 2 60gb SSD's and run them in RAID 0, that way you can get speeds up to 875mbs in sequential read and write.
To answer your second question yes you can transfer all your files over and even clone your current drive and copy the exact disk image to the new SSD/SSD's without a reinstall. Do not try and copy game folders to another storage device and then copy over becuase the registry values will be incorrect and can cause errors with the game your trying to run, its better to either make a clone image or do a fresh reinstall