I would say avoid the 840 TLC, as it's the first TLC SSD in large scale production so TLC's longevity is still unproven. Intel 520, Samsung 830 are excellent options. Aside from that Kingston HyperX is also a good option. Really any SSD will be a huge improvement over a HDD. Between SSD's you may not notice a difference unless you're doing really intensive read/write. Just check reviews before you buy and make sure there aren't any widespread issues.