Depends on what resolution you are planning to be playing on.
If you have a 2560x1600 Monitor, or are running games across 3 monitors eventually you will need more memory, maybe run it in SLI.
But, if you are running at 1920x1080 or lower than 3GB is more than enough for a long time to come, this is because the speed of the RAM is fast enough to refresh without filling up it's entire cache.
So yes, the GTX590 will be enough for a long time to come unless you are running at incredibly high resolutions.