Stampeders questions are the most important.
I agree that 240-250 Gb is the sweet spot for price at the moment with the 440-512 size starting to drop to a decent price. My three home desktops have all been retrofitted with SSD. They run W7 and W10
All mine are Samsung 840 drives bought at various times ( and prices!) Intel and Crucial both have a good reputation for these devices.
Transferring the system onto SSD does need some preparation if you have a machine with a big drive and lots of data to determine what apart from the OS should be moved...the SSD manufacturers try to make it sound simple but as you do not want to put any really volatile data ( like TEMP or the Browser cache ) onto the SSD it takes some thought.