SSDs make loads time a lot faster. It has become a custom to use an SSD for your OS and main applications while having a traditional HDD for storage and other apps. I would go with a 7850 for the GPU if you can stretch it. You said casual gaming but the FPS increase of the 7850 over the 7770 is well worth it.
I dont really understand how they work. lol. are they worth it?
Yes. In my opinion they are. Overall snappiness of the system will improve. As the above posted most people load the OS and Apps they use frequently. Basically what a SSD does is it gets rid of the HDD bottleneck of waiting for things to load. It's worth it if you can fit it into your budget .