Difference between 970fx and 990fx:970
Codenamed RX980
One physical PCIe 2.0 ×16 slot, one PCIe 2.0 ×4 slot and three PCIe 2.0 ×1 slots, the chipset provides a total of 22 PCIe 2.0 lanes and 4 PCIe 2.0 for A-Link Express III solely in the Northbridge
HyperTransport 3.0 up to 2400 MHz and PCI Express 2.0
13.6 Watt TDP
Southbridge: SB950/SB920
990FX - Latest one.
Codenamed RD990
Four physical PCIe 2.0 ×16 slots @ x8 electrical which can be combined to create two PCIe 2.0 ×16 slots @ x16 electrical, one PCIe 2.0 ×4 slot and two PCIe 2.0 ×1 slots, the chipset provides a total of 38 PCIe 2.0 lanes and 4 PCIe 2.0 for A-Link Express III solely in the Northbridge
HyperTransport 3.0 up to 2600 MHz and PCI Express 2.0
AMD CrossFireX supporting up to four graphics cards
19.6 Watt TDP
Southbridge: SB950
Enthusiast discrete multi-graphics segment
So in short if you don't do SLI or CrossfireX then get 970fx because it saves money. Otherwise go with 990fx. Also 990fx has higher chance of supporting new cpus but the chance is nearly 0