The Radeon HD7850 only requires a "500-watt" power supply if it's one of those cheap Chinese models that lies about its capacity. A decent power supply with 30 or more amperes of current available on the +12 volt header isn't going to complain. The Silverstone power supply in the Dorm PC 2.0 build can provide 36 A at +12 V. No worries.
Note that you could spend a bit less money, have much more expandability (4 PCIe slots vs. 1) and have many more alternatives available if you went with a Micro-ATX build rather than Mini-ITX.
i7-2600K, H70, P8P67-M Pro, 16 GiB, HD7950, SSD, 3 HD, Blu-ray, X-Fi Ti Pro, Antec P182, S75CF, 3007WFP+2001FP, RK-9000BR, MX518
i5-3570K, GeminII-S524, P8Z77-M Pro, 16 GiB, HD7770, SSD, 2 HD, Blu-ray, InifiniTV4, NSK2480, 55" TV; Asus UX32VD