Generally the rule is for gaming, upgrade the graphics card before the CPU. A 7870 is decent but it's starting to get old and when it came out it was only really a mid-range card. Upgrade to something like a 970 or wait for AMD's 300 series to come out if you'd prefer to stay with AMD.
Your processor should last you at least another few years, so no need to worry about that just yet.
Edit: Also, what's your budget? The 960 just came out and that's a solid choice for about 100 dollars cheaper.