Depends if you can be bothered with reinstalling operating system + games.
Since hdds aren't dear, would make more sence get an ssd, depending on your budget i would look at either a 256GB/512GB Samsung 840Pro SSD and get 2TB Seagate/WD Caviar Black for 80$ or so.
Keep the os/games you love on the SSD and configure internet downloads/steam backups to the HDD.
Also there's a few tweaks that are necesary for an SSD that Windows does not do by default. Think they would, not at the moment so its up to you.
Answers are here: http://www.thessdreview.com/ssd-guides/optimization-guides/the-ssd-optimization-guide-2/
Also Samsumg provide their own software to make these changes...