Any standard 2.5 will "fit". Yes, changing to an SSD is the single most valuable upgrade you can do for your machine. Its very simple to do as far as physically changing them , just open your machine and change them. As far as installing a O.S. on it , its pretty simple too. I'm enclosing a couple of links to a step by step tutorial on how to do that. Having said THAT, you can usually find SSD "bundles" that come with software/hardware needed for that. I did it myself and saved a few bucks:)! I DEFINITELY suggest spending the money on it rather than a new lappy, depending on its otherwise general shape of course. You'll have a faster, smoother machine with any SSD but I will not recommend any particle brand. My M4 Crucial is and has been spectacular. The Samsung Pro series is the fastest out there now, pretty much, and has been getting good word of mouth.
http://www.sevenforums.com/tutorials/149969-ssd-install-transfer-operating-system.html
G74Sx BBK-8 Crucial M4 128GB.SSD/W.D.500Gb.@7200 HHD 16GB. RAM Stock otherwise.
An End,even with Terror is better than Terror without End. F.Nietzsche