Why would you want a discontinued Rosewill Xtreme Series RX850-S-B 850W from 8 years ago? It's an old group regulated circuit design, Taiwanese capacitors, hardwired output cables, lower efficiency shorter warranty.
The EVGA SuperNOVA 850 B2 (110-B2-0850-V1) is superior to that Rosewill Xtreme in every way. Modern DC-to-DC circuit design, semi-modular output cables, all Japanese capacitors, higher efficiency and longer warranty.
System Power Supply Requirements for a single AMD Reference Design Radeon R9 280X
• a minimum of a 600 Watt or greater system power supply is recommended. (Minimum system power requirement based on a PC configured with an Intel Core i7 3.2GHz 130 Watt TDP processor.)
• the system power supply must also have a maximum combined +12 Volt continuous current rating of 37 Amps or greater
• the system power supply should also have at least one 75-Watt 6-pin and one 150-Watt 8-pin PCI Express supplementary power connectors. There are some non-reference design cards that require two 150-Watt 8-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) is the most critical factor.
Overclocking of the CPU and/or GPU(s) may require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.
EVGA SuperNOVA 850 B2 (110-B2-0850-V1)
• OEM: Super Flower
• maximum combined +12 Volt continuous current rating: 70 Amps <===== Way more than sufficient
• two 75-Watt 6-pin and four 150-Watt (6+2)-pin PCI Express supplementary power connectors <===== More than sufficient
• Ambient Temperature Maximum (i.e. without derating): 50°C
• Semi-Modular Output Cables: Yes
• Passes Official Intel Haswell Compliance Test: Yes
• 80 PLUS BRONZE Efficiency Certification
• 5 Year Limited Warranty