If you're ready to bring your PC into this decade, here are some great deals in a variety of SSDs. The 240GB Kingston would be great for your OS and commonly-used apps, while the 840 EVO 1TB drive offers fantastic performance and enough space to store most, if not all of your files.
Kingston 240GB SSD Now ($105) | Amazon
Crucial M550 512GB SSD ($285) | Amazon
Samsung 840 EVO 1TB SSD ($429) | eBay