Yes, first of all you will need a more powerful power supply. You can look for a corsair
, both are great brands or a thermaltake would do fine too. Just make sure the power supply has a bronze or above quality which determines the actual power consume. A 650 Watt PSU would be fine but always have room for future possibilities and I recommend a 750W + PSU, in case you may want to crossfire your gpu with another one.
As for the gpu, never ever, go for a dual 7850. Always head for the fastest single gpu you can afford. At the very least go for hd7870xt versions. Remind you, many games do not support crossfire/SLI configurations and you may suffer FPS rates as well as Frame Latency due to dual gpu. And it would create more than twice as heat a single gpu would generate, so if you don't have a big enough case with enough air circulation never go for dual gpu's.
I strongly recommend you to buy a single 7970, in case you may want to buy a higher res. screen, a 7970 can save you from all the trouble a game might bring
Even with Crysis 3 at max. the card creates wonders.
But, I would wait a couple of months to see the new series that AMD will bring, the rumor says that new series will be a kick [bleep]
just saying may be you may want to invest in a better and a newer gpu, when they come out. Still, wait so that 7970's price will drop when the new serie kicks in. But dont try upgrading your gpu before you upgrade your PSU.
About that FPS worries you have, with any card you have, just activate the V-Sync option from graphincs settings in-game and it must solve the problem.
A bit late it seems for the answer but hope not too late
peace just pm me if you have any further questions about hardware and gaming