PCPartPicker had no complaints with regard to wattage (and component compatibility), and neither did the several reddit/Tom's Hardware browsers who gave me feedback on my earlier part selections. However, on the physical box of the GIGABYTE R9 280, there is a small disclaimer in yellow at the bottom noting that the minimum power supply wattage required is 600W (for a configuration with an i7). My question is, will I encounter any difficulties wattage-wise given my i5 (which is not far off some of the lower-end i7's)? I do not want to assemble the system only to have damage done to expensive components as a result of miscalculations.
No you won't have any problems at all , You have a great quality PSU.
The card will never draw more than 250W under full load , the disclaimer on the box is just because they know that some people use cheap quality psu so they say a number just to keep people on the safe side of things so you needn't worry as the numbers on the boxes don't mean anything