I guess it depends on what you're doing on that PC:
-The 6300 is a proficient processor for gaming though there are definitely faster options on the market
-Your 7870 is a good budget card and adding another one would improve your overall GPU performance but the 2GB of video memory is pretty limiting and crossfire doesn't double your memory so you're not really addressing that problem with adding another 7870
-600W is more than enough for a single GPU unless you're using the most demanding cards on the market - I would prefer a Corsair in terms of quality, performance and reliability (Seasonic is the best still, but they're also more expensive). I would say replace your power supply down the road
-A solid state drive for your operating system is the most impressive day to day usage performance upgrade you can make, but it doesn't make a noticeable difference in multiplayer gaming so if your goal is a better gaming experience you can skip it. I would still recommend it down the road, makes your computer feel zippy by decreasing your boot time and your load times - I like the Samsung 850 Evo: good performance and reliability for not a huge price tag
If your goal is a better gaming experience, I would look at a selling your current card and upgrading - if you want to stick with your budget and AMD, you're looking at a R9 380. Go with a 4GB model like this one:
http://pcpartpicker.com/part/msi-video-card-r9380gaming4g
I personally prefer Nvidia - if you don't mind jumping ship, you're looking at a 970. I would recommend throwing in an extra $20-$30 to get one of the overclocked options. I have the 770 version of this card - it's a frickin animal:
http://pcpartpicker.com/part/evga-video-card-04gp43975kr