I'm about to buy this....

Yeah, you need a PSU in there:
PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-4670K 3.4GHz Quad-Core Processor ($219.99 @ Newegg)
Motherboard: MSI Z87I Mini ITX LGA1150 Motherboard ($119.98 @ SuperBiiz)
Memory: G.Skill Sniper Series 8GB (2 x 4GB) DDR3-1866 Memory ($68.00 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.98 @ OutletPC)
Video Card: Sapphire Radeon R9 280X 3GB Video Card ($299.99 @ Newegg)
Case: BitFenix Prodigy (Black) Mini ITX Tower Case ($74.98 @ OutletPC)
Power Supply: Corsair CX 500W 80+ Bronze Certified Semi-Modular ATX Power Supply ($39.99 @ Newegg)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Total: $897.90
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-12-20 18:04 EST-0500)
 
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