Yeah, that 450W PSU that comes with the SG05 is plenty. You can check with a power supply calculator.
Keep in mind the recommendations from PSU calculators assume everything is at 90% load and the PSU is at 50% load. (ie, for a system that will pull 250W under 90% load, a PSU calculator will suggest a 500W PSU)
Make sure you pay attention to the length of the GPU you buy. According to the anandtech review I linked earlier, 10.2" is the MAX.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod