An SSD is totally justified now that the prices have fallen so much
The thing to remember is you are probably going to want both SSD and mechanical drives, Theres a huge benefit to putting your OS/Games on the SSD but practically no benefit at all in putting media files like mp3s/movies onto one. Media files are streamed off the disk at a set rate from beginning to end which is well within the capabilities of a mechanical drive, so if you have many terabytes of horse porn its not worth paying the premium of SSD to store them
However SSDs are fantastic as fast random access which is why they are so good, I dont know about your PC but my windows folder contains over 100,000 different files and having your OS and programs on SSD can really make everything seem a heck of alot snappier, its not mechanically moving a drive head back and forth over the disk surface trying to get to all the various files needed, you click a button and programs are loaded so fast that you've barely lifted your finger from clicking
Realistically most people would probably want to aim for a 128gb drive, windows 7 takes about 30gb, that leaves 100gb for your apps and games.
edit: go for bigger if you want to run VMs ofc as those could sit on the SSD