If you're gaming at 1080p then the HD 7870 is likely holding up very well. I'd add the SSD now and then add a new single GPU down the road as funds permit.
I recommend you getting crossfire if you want an FPS increase.
An SSD is only good if you want to load software or boot up windows faster. Which means it will not give you any extra fps.
How ever, personally I would recommend getting a GTX 970. It's all up to you though, I'm just giving advice.
Crossfire will yield better performance, BUT I would NOT recommend it for you for 2 reasons.
1. Your PSU will not be good enough.
2. Your motherboards second PCI-e slot will only be running at x4 speeds. Which will actually cause a lot of issues and probably cause microstutter.