what am i missing?

GIGABYTE GA-990FXA-UD3 AM3+ AMD 990FX SATA 6Gb/s USB 3.0 ATX AMD Motherboard
AMD Sempron 145 Processor
(2) Corsair RM Series 850 Watt ATX/EPS 80PLUS Gold-Certified Power Supply - CP-9020056-NA RM850
P3 International P4400 Kill A Watt Electricity Usage Monitor
Kingston Technology HyperX 8 GB (2x4 GB Modules) 1600 MHz DDR3 Dual Channel Kit (PC3 12800) 240-Pin SDRAM KHX1600C9D3K2/8GX

using crates


(2) Powercolor Radeon R9 280X OC 1030MHZ 3GB 6.0GBPS GDDR5 DVI HDMI 2XMINIDP PCI-E Video Card
(1) Asus MATRIX-R9280X-P-3GD5 Radeon R9 280X DDR5 3GB256-bit PCIE 3.0 DVI/DP/HDMI
(1) GIGABYTE Radeon R9 280x OC GV-R928XOC-3GD REV2
(1) XFX R9-280X-TDBD AMD Radeon R9 280X 3GB GDDR5 2DVI/HDMI/2Mini DisplayPort PCI-Express Video Card


I'm confused on the risers
Expansion Slots: 2 x PCI Express x16 slot running at x16 ,1x PCI Express x4 slots, 2 x PCI Express x1 slot, 1x PCI slots

do i need (5) 16x1 risers or
(2) 16x16, (1) 16x4, and (2) 16x1


Im planning on running ubuntu on a memory stick since i read that's the best way to go!