There is not much of a difference other than possibly the connection cables when getting a desktop or laptop version of an SSD. You can hop onto Amazon, Newegg, or whatever retailer/etailer of choice you have and see what is out there. I got my Samsung 840 EVO from Amazon and I like it.
Some people may tell you to transfer the content from the HDD to the SSD, but it all depends. If there is a lot of content you want to transfer over, then go for it. But if there is not much of anything, you could save that data to a memory card or USB and do a clean install of the OS onto the SSD. Then format the HDD and put it as a secondary drive.
Like I said, it depends on what you prefer to do. I always like a clean install. In my case I have all of my discs, Steam keeps my saves, and anything else I need I can throw on an external hard drive.
If you want to, you may just end up transferring the OS over to the SSD from the HDD. But remember that if the HDD has a bunch of content on it that is past the amount available on the SSD, then you would end up with a big mess on your hands.
Also, just a bit more info, the size of the HDD or SSD is not the size advertised. There is a certain amount of space that is set aside and the way memory is comprised, it just is not the same. So, for all intents and purposes, a 250 GB SSD may only have 220 GB of space you can use.
http://www.amazon.com/Samsung-2-5-Inch-Internal-MZ-75E250B-AM/dp/B00OAJ412U/ref=sr_1_1?ie=UTF8&qid=1433986352&sr=8-1&keywords=samsung+850
is a pretty good hard drive and for the price, may be somewhat you are looking for.