How long do you plan to use the GPU?
If only for a few months then buying a complete new build - I'd say stick with your 480.
If using it in the current build and moving it over to a new $500 build - get the best performing card you can afford (under about $300).
If using it couple months and moving to a $1500 build - Go for something like a 280X (~$250+) up to a 970 (~$400)
If no upgrade plans in next couple years I'd look for a great price on something like the 280X (saw couple this week on for $240 after rebate) or a 760/280 if you can get it for well under $200, or a 280X equivalent like a 680/770.