its not due to 512Mb ram , Lumia 620/720 have 512 Mb ram and they do have equalizer , this will never come with Os update , its all dependent upon hardware and firmware update + audio update . Lumia 520 lacks equalizer hardware :sad:
just Update windows 8.1 preview, Audio Equalizer Available for 720 lumia Don't know about 520 Because.
I think There is no supporting hardware available in that device.