So, for a PSU, I'm not sure what to get or how many watts it takes. There's this 500W PC Power and Cooling power supply. http://www.newegg.com/Product/Product.aspx?Item=N82E16817703015
OR this 550W one http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004
OR this 520W one http://www.newegg.com/Product/Product.aspx?Item=N82E16817139001
OR this 650 one http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005
Your setup doesn't use much power. You can run that config on a good-quality 350W PSU, because your graphics card draws less than 75 watts of power.
Notice that 8600GT does not even have a 6-pin PCIe power connector. Only video cards that draw more than 75 watts of electrical power are required to have the 6-pin.
Also, you realize that Gigabyte 790X board is an ATI Crossfire motherboard, right? http://www.gigabyte.us/Products/Motherboard/Products_Spec.aspx?ProductID=3031 You will NOT be able to run two nVidia video cards on it in SLI, so you can forget about adding a second 8600GT if that's what you are planning to do.
To run two video cards on a Crossfire motherboard, you need two ATI video cards with the same GPU.
CrankBrothers crank brothers bicycle Power Pump Pro Black with gauge compact, AEM Analog Electrical Oil Temperature Gauge 2 1/16" Dia Black Face 30-5140, 8 GAUGE AWG BLACK WIRE 50 FT POWER\GROUND HIGH QUALITY, DB LINK GW8BK250Z POWER WIRE (8 GAUGE; BLACK; 250 FT), 8 GAUGE AWG WIRE BLACK GROUND/POWER NEW BY THE FOOT FT , 20 FT RED 8 GAUGE POWER WIRE + 5 FT BLACK GROUND + FUSE, BULLZ AUDIO BPG0.50BK Car Audio 1/0 Gauge 50 FT Power Wire Ground Cable - Black, DB LINK GW8BK250Z Power Wire (8 gauge; black; 250 ft), BULLZ AUDIO BPG0.50BK Car Audio 1/0 Gauge 50 FT Power Wire Ground Cable - Black, BRAND NEW BULLZ AUDIO Car Audio 1/0 Gauge 100 FT Power Wire Ground Cable - Black, BULLZ AUDIO BPG0.50BK Car Audio 1/0 Gauge 50 FT Power Wire Ground Cable - Black, NEW DB LINK GW8BK250Z Power Wire (8-gauge; black; 50 ft, THE WIRES ZONE 8 GAUGE 250' FT WIRE POWER/GROUND BATTERY CABLE BLACK TRUE GAUGE , SCOSCHE-EFX REVOPAK8 8-GAUGE REVO SERIES SILVER/BLACK SINGLE AMP POWER & AUDIO, SCOSCHE-EFX REVOPAK4 4-GAUGE REVO SERIES SILVER/BLACK SINGLE AMP POWER & AUDIO, NEW DB LINK STGW4BK100Z SOFT TOUCH POWER WIRE (4-GAUGE, BLACK, 100-FT ROLL), NEW DB LINK STGW8BK250Z SOFT TOUCH POWER WIRE (8-GAUGE, BLACK, 250-FT ROLL), 20 FT RED 4 GAUGE POWER WIRE + 5 FT BLACK GROUND + FUSE, 4 GAUGE AWG WIRE BLACK GROUND/POWER NEW BY THE FOOT FT, BULLZ AUDIO BPG0.50BK Car Audio 1/0 Gauge 50 FT Power Wire Ground Cable - Black, SOUND QUEST SQVLP8B 8 GAUGE CCA 250FT SPOOL BLACK POWER CABLE / WIRE - VL SERIES, SOUND QUEST SQVLP0BK CCA 0 GAUGE CAR AUDIO POWER / GROUND WIRE MATTE BLACK, 25FT 16 Gauge Black Extension Cord AC Power Cable 2PK, BULLZ AUDIO BPG0.50BK Car Audio 1/0 Gauge 50 FT Power Wire Ground Cable - Black, 10 Pack 16 Gauge awg 10' ft POWER EXTENSION CORD Black, BRAND NEW BULLZ AUDIO Car Audio 1/0 Gauge 100 FT Power Wire Ground Cable - Black, Scosche Black Flux Power Cable 10Ft 8 Gauge 022FP8BM-10 Marine 10', Scosche Black Flux Power Cable 10Ft 4 Gauge 022FP4BM-10 Marine 10', Aapex BLACK 0 Gauge AWG Power Wire Cable Stranded Copper Positive - by THE FOOT, Aapex 25' FT BLACK 0 Gauge AWG Power Wire Cable Stranded Copper Positive 25 FEET, NEW 20FT 10 GAUGE JSC RED/BLACK ZIP WIRE POWER CABLE, 5 FT PER BID STINGER 16 GA GAUGE PRO BLACK POWER WIRE, 5 FT PER BID STINGER 18 GA GAUGE PRO BLACK POWER WIRE, 5 FT PER BID STINGER 12 GA GAUGE PRO BLACK POWER WIRE, 5 FT PER BID STINGER 14 GA GAUGE PRO BLACK POWER WIRE, Banks Power Gauge Panel Aluminum Black Two 2 1/16" Each 63002, DB LINK STGW4BK100Z SOFT TOUCH POWER WIRE (4-GAUGE, BLACK, 100-FT ROLL), Aapex 50' 50 ft 25 RED & 25 BLACK 8 Gauge AWG Power Wire Cable Stranded Copper, BULLZ AUDIO BPG0.50BK Car Audio 1/0 Gauge 50 FT Power Wire Ground Cable - Black, BRAND NEW BULLZ AUDIO Car Audio 1/0 Gauge 100 FT Power Wire Ground Cable - Black, STINGER S4GBSBB CAR AUDIO GOLD 4 GAUGE POWER WIRE BLACK SPADE TERMINALS 25 PACK, STINGER S4GBSPTBB CAR AUDIO 4 GAUGE POWER WIRE BLACK SPADE TERMINALS 25 PACK, PYRAMID 4 Gauge Black Car Stereo Power Ground Wire 25ft, (1) FOOT 1/0 Gauge Ga AWG Black Tsunami OFC Oxygen Free Copper Power Wire Cable, Banks DynaFact Mechanical Boost Pressure Gauge 2 1/16" Dia Black Face 63435, BULLZ AUDIO BPG0.50BK Car Audio 1/0 Gauge 50 FT Power Wire Ground Cable - Black, KALIBUR 12 GAUGE RED/BLACK ZIP WIRE POWER SUPPLY CABLE, 25FT 14 Gauge Black Extension Cord AC Power Cable 2PK, Hornby R044 Point Motor Passing Contact Switch Black 00 Gauge Power & Control Ac, REVOPAK8 8-Gauge Revo Series Silver/Black Single Amp Power & Audio Kit,
