we are all obsessed with buying new shiney shiney stuff
and yea u make sense, because i bet we all say "well if i just get this....."
u just running one monitor? i think you should keep the 580..
i guarantee if u get the 590, you will think "why didnt i keep the 580 and save my money for the GTX6xx series card" and that will go on forever
im limiting myself to getting a GTX570, it meets all my needs, and will do so for the forseeable future..
( and my missus wants a shiney ring for her birthday )
keep the 580 and if u REALLY feel that money burning a hole in your pocket.. buy a new case lol