I would suggest to start out with a minimal Debian or Ubuntu image, check what RAM you have left with a bare system and then slowly go from there, install X and maybe try Xfce then check again what RAM resources you have left and see what applications you can run. It's also important to have a sdcard that has good performance with random I/O if you plan to run desktop type applications... see http://forum.pine64.org/showthread.php?tid=497 or http://forum.pine64.org/showthread.php?tid=376 for example or the Debian image with LXDE (see http://lxde.org/ )
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